The Emhryology of Chlamydoselachus 587 





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Text-figure 25 

 Median sagittal section (?) through the blastoderm, subgerminal cavity, and periblast of an egg 

 of Chlamydoselachus in a late blastula stage. The zone of periblast is evidently not shown 



in its entirety. 

 Drawing by Bashford Dean. 



in the early cleavage stage represented by Text'figure 24. Judging from Text-figure 25, 

 the late blastula also is remarkable for the depth and breadth of the zone of yolk nuclei, 

 which is evidently shown incompletely. Two other drawings of a late blastula, not 

 reproduced, are very similar to the one shown in Text'figure 25, and were probably 

 made from the same set of sections. These drawings all studied together indicate that 

 in these early stages the extent of the germinal area is greater than the portion of it which 

 is cut up into blastomeres. 



It is my belief that all these drawings were made by Dean. On the boards on which 

 they are mounted are notes in Dean's handwriting. The minute details in which the 

 drawings abound are executed in Dean's characteristic style — according to his former 

 students to whose attention they have been called. 



The internal structure of the late blastula described by Nishikawa (1898, pp. 96-97) 

 is evidently similar to the one studied by Dean and portrayed in my Text'figure 25. 

 Indeed Dean's drawing was probably made from Nishikawa's blastoderm and from 

 a section near the one shown in Text'figure 23. Nishikawa's description is as follows: 



The next stage was a blastula, with a distinct segmentation cavity, whose floor was 

 bounded by what has been termed "periblast" with finely granular yolk, and merocytes, with 

 vacuolated cytoplasm, due perhaps to the dissolution of the contained oil drops, and many 

 nuclei. One end of the blastula was thicker than the other, and is evidently the "embryonic 

 end" of Balfour, and the "anterior end" of Riickert. On the surface of the blastoderm the cells 

 are arranged epithelially. Most cells of the blastoderm are rich in yolk granules, but at the 

 bottom of the blastoderm they have only a coarsely granular cytoplasm. The blastodermic 

 cells are added from the periphery by the merocytes with fine yolk granules, as may be seen 

 from cut 1 [Text-figure 23 herein] which has been composed from two consecutive sections. 

 I have also found a cell simply resting on the floor of the segmentation cavity; but I axnnot 

 say for certain whether it originated from the periblast or from the blastoderm. 



