490 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxix. 



EARLY STAGES PBEPARATORY TO INVAGINATION. 



Fig. 84 is a normal U'pe in which the cells are beginning to move 

 away from the peribla.st, to crowd together in the upper part of the 

 blastoderm, and to leave a subgerminal cavity (,s, g. e.) between them 

 and the periblast. The line marked ,/' is, in this and the following 

 sections, the lower limit of the cells. The outermost cells of the blasto- 

 derm have flattened until the}' make a very thin skin-like la3^er. The 

 periblast is comparatively free from 3^olk granules and is here drawn 

 after nature instead of semi-diagrammatically. 



The second type is represented in tig. 85. The cells are densely 

 crowded, the periblast depressed, and the subgerminal cavity {s. g. c.) 

 is very large. The periblast is very thick and yolk-laden, and so 

 heavily stained that onl}' one nucleus could be made out. 



Fig. 86 illustrates the thick-ended tj^pe. In this section the cells 

 are not so closely crowded as in the preceding, but a distinct subger- 

 minal cavity is formed. The very distinct periblast contains many 

 large nuclei, and on the left is separated from the blastoderm by a 

 sharp reentrant angle. A very large binucleate cell is .shown, and 

 nearby two others are found. On the left is shown a cell of ordinary 

 size. 



Fig. 87 represents the high-arched type like fig. 83, which has begun 

 to flatten out in preparation for the next stage. This flattening is 

 probably responsible for the small subgerminal cavity. The periblast 

 has many large nuclei. Two blastomeres shown indicate the size of 

 the cells at this stage. 



LATE STAGES PREPARATORY TO INVAGINATION. 



Of these only two will be shown. Fig. 88 is the normal teleost 

 structure for this stage. The cells are all closely crowded into a high- 

 arched band, having a large subgerminal cavity {s. g. c.) below. The 

 periblast is here filled with 3'olk and contains many flattened nuclei. 

 The blastoderm has begun to spread out over the yolk, and the section 

 in fig. 88 is 25 per cent longer than that in fig. 84. 



Whether the slight difl'erence in shape of fig. 89 in comparison with 

 fig. 88 is due to contraction caused by the killing fluid or whether it 

 is due to descent from a form like fig. 86 would be hard to decide. 

 Possibly the latter idea is correct. The periblast is filled with yolk 

 fragments, and the nuclei are very much flattened. 



VII. THE PERIBLAST. 



The origin of this layer, together with man}' of its peculiarities of 

 structure, has been noted in the descriptions of the plates. It is not 

 my intention to go now into any extended discussion of its formation 

 and fate. However, it will be well to describe briefly the various 



