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The Origin and Development of the Anterior Lymph-Sacs in the Sea-Turtle (Thalasso- Chelys Caretta)

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The Origin and Development of the Anterior Lymph-Sacs in the Sea-Turtle (Thalasso- Chelys Caretta) The Origin and Development of the Anterior Lymph-Sacs in the Sea-Turtle (Thalasso- chelys caretta). By E. R. Van Der Jagt, University of Iowa. With 6 Text-figures. INTRODUCTION. IT requires only a hasty inspection of the literature on the lymphatic system to show the unsatisfactory state of our know- ledge concerning its development. In the first place, the onto- geny and phylogeny stand in apparent conflict. There are, moreover, few features of its development upon which there is any agreement among the various investigators who have con- tributed to this field. As a reworking of the subject, the author was guided in choosing the turtle, not only because of the con- troversy over the origin of the anterior lymph-sacs, but also because in this form the embryonic anlagen of the system are easily distinguished from the blood-vessels. In 1911 G. S. Huntington published a paper on the origin of the lymphatic system in reptilian embryos. Since his results and conclusions did not present fully all the evidence, or agree with the results of later investigators, E. L. Clark (1912), Kampmeier (1912), Stromsten (1912), Professor Stromsten sug- gested to the writer the advantages of repeating his work, but limiting the investigation to the turtle. For the completion of this work, I am indebted to Professor Stromsten for his valuable suggestions and helpful criticism during the investigation. MATERIAL AND TECHNIQUE. The embryos of Thalassochelys caretta used in this work were collected by Dr. Stromsten at the Marine Laboratories of the Carnegie Institution of Washington at the Dry Tortugas, 152 E. R. VAN DER JAGT Florida. They were fixed in a mixture of chromic acid, glacial acetic, and 40 per cent, formaldehyde. In the advanced stages the animals were narcotized before fixation in order that the delicate mesenchymal tissue might not be injured by the move- ments of the embryo when first placed in the fixing solution. Throughout the preparation of sections for final mounting care was taken to prevent shrinking or tearing of the tissues. Sections were cut from 10 to 20 micra in thickness, and were stained with Delafield's haematoxylin, iron haematoxylin, or in to to with alum or borax-carmine. They were counter- stained with orange-G (slightly acidulated), eosine-aurantia- orange-G, and Mallory's connective-tissue stain. The observations reported are based wholly upon the study of sections of embryos and camera lucida drawings, which were made serially of all stages in the region under investigation. The drawings were made on cellophane and the cellophane method of reconstruction was used.1 It is the purpose of this paper to present results of a study of development of the lymph-sacs as well as a determination of their anlagen. By the study of serial sections it is shown that independent lymph-spaces occupy the region of the developing sac, and that these spaces by fusion aid in the formation of a plexus which later becomes transformed into the jugular sac. OBSERVATION AND DISCUSSION. It is evident from the literature (see bibliography) that there are three views concerning the development of the lymph- hearts : 1. The lymph-hearts arise from the veins at various centres of radiation and by continuous elongation and fusion form the hearts. 2. The lymph-hearts are derived from the embryonic venous system either by a direct transformation of certain of its channels or by the fusion of multiple derivatives which have become detached from it. i Van Der Jagt, E. R., "An improvement in the technique of reconstruc- tion work by the use of cellophane", 'Science', Dec. 1931. LYMPH-SACS OF TURTLE 158 3. The lymph-hearts arise by the confluence of mesenchymal spaces which invest and communicate with the capil- laries. The present observations and studies favour the third view which holds that the lymph-hearts and sacs are initiated in development by the vacuolation of the mesenchyme. They are not a product of the veins either by centrifugal growth or the fusion of detached venous elements. The endothelial lining of the lymph-sacs is a gradual differentiation from the mesen- chymal tissue. The jugular lymph-sacs of the turtle present a considerable range of variation both in the advanced structure and in the details of their development. The variations, however, are neither so great nor so numerous as in the higher forms. These variations exist not only in different embryos but even upon opposite sides of the same embryo. Besides these individual variations, one often finds certain variations due to develop- mental conditions. Not only do certain embryos show advanced development in certain tributaries, and retarded conditions in others, but they differ in even more minor details, as for example, in the number and arrangement of secondary tributaries and their anastomotic conditions. Although a general principle of development can be established, the actual mode of origin of certain of the veno-lymphatic anlagen of the jugular lymph- sacs cannot be stated definitely owing to the variability in development of these veno-lymphatics in conjunction with the main venous channels and their tributaries, as well as the variable manner in which they fuse together. The active period of lymph-sac development is comparatively short. Embryos of seventeen to twenty-five days represent the important developmental stages. We may divide the history of the development of the sacs into three periods. 1. The formation of mesenchymal spaces in the anterior cardinal regions. 2. The enlargement and fusion of spaces with each other and with the venous tributaries to form the veno- lymphatics. 3. The confluence of the veno-lymphatics to form the sac, 154 E. R. VAN DER JAGT with the transformation of the mesenchyme cells to endothelial cells. 1. The Formation of Mesenchymal Spaces. This period of development brings up the point immediately as to the source and formation of lymphatic anlagen. A careful study of the stages belonging to this developmental period points out to the observer three facts of major importance: First, the compactness of the mesenchyme at an early stage, as shown in Text-fig. 1. In the lymph-sac area of embryos at the end of the second week of development the mesenchyme is very compact, showing slight signs of vacuolation. This area is bounded by the anterior cardinal vein and its dorsal branches, the wide-meshed superficial plexus, and the ventrolateral branches of the cardinal vein. The nuclei of the mesenchyme cells in this area still show the characteristic features of mesen- chyme structure as compared with the well-defined endothelial cells lining the veins in this region. Embryos in the seventeenth day of development show marked changes. The mesenchyme in the dorsolateral region shows a loosening up of the cells. The vacuolation of the mesenchyme is forcing the cells apart. The intercellular spaces are increasing in size and number. The nuclei become widely separated and the cells are only connected by thin strands of protoplasm (Text- fig. 2). Secondly, the formation of isolated lymphatic spaces as the result of increased vacuolation is an important step in the forma- tion of the lymph-sacs. The rapid formation of tissue spaces cause the thin protoplasmic processes to break or disintegrate. These fine processes or strands may be seen projecting out into the lumen of the cavities thus formed (Text-fig. 3). The cavities are not lined by a definite endothelial wall but by the former mesenchyme cells of the individual spaces. The spaces are always present in the region of the developing lymph-sac and are found in direct connexion with the anterior cardinal tributaries. As these two types of anlagen unite to form the veno-lymphatics, vacuolation continues, forming again small spaces in direct connexion with the established veno-lymphatics. LYMPH-SACS OF TURTLE 1S1 This process continues even after the veno-lymphatics begin to fuse. The continual formation of these small isolated lymph- spaces in connexion with the fusing veno-lymphatics, deter- mines the ultimate lymph-sac. The spaces occurring in the lymph-sac region are small in comparison to the lymph-spaces developed in connexion with other lymph-channels. This may TEXT-FIG. 1: Loggerhead turtle, fourteen days; series 516, X400. Photomicrograph of a section taken through the region of the anterior cardinal vein to show the compactness of the mesenchyme, which is comparatively free from vacuolation at this stage. The nuclei still show the characteristic feature of mesenchyme struc- ture as compared to the well-defined endothelial cells lining the veins in this region. Ant. Card. V., anterior cardinal vein; Mes., mesenchyme; End. V., endothelium of the vein. TEXT-FIG. 2: Loggerhead turtle, seventeen days; series 563, X400. Photomicrograph of the section through the anterior cardinal vein. The mesenchyme in the dorsal and lateral region begins to appear vacuolated. This vacuolation is forcing the mesen- chymal cells apart, causing an increase in size and number of intercellular spaces. Thenuclei are becoming more widely separated and the cells show connexions through thin protoplasmic strands. Ant. Card., anterior cardinal; End. V., endothelium of vein; I.C.S., intercellular spaces; Mes., mesenchyme. be due to the compactness of the mesenchyme or the more rapid reabsorption of the lymph in this region. Thirdly, the appearance of the mesenchyme cells in this region is in marked contrast to that of the cells of an embryo in which vacuolation has not occurred in the lymph-sac region. With the appearance of the spongy mesenchyme the shape of the 156 E. B. VAN DER JAGT cells is markedly altered. The protoplasm becomes drawn out to thin protoplasmic strands and only a limited amount of cytoplasm remains to surround the nuclei.
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