72 CHIM^EROID FISHES AND THEIR DEVELOPMENT. 



to as passing slightly to one side of the sagittal plane shows favorably the 

 thickening of the ectoderm at the side of the medullary plate and its inbending. 

 We here observe also the reduced size of the segmentation cavity, the thickening 

 of the cell mass roofing the archenteron, and the thinning out of the mass of cells, 

 pm, forming its floor. 



We may at this point consider conveniently the general bearing of the process of 

 early gastrulation in the Chimseroid. We have seen that: 



(a) In an early stage an archenteron was present (fig. 63), whose ventral wall 

 was composed of cells and whose axis was at right angles to the surface of the 

 blastoderm. 



(<$) In a second stage, the area of the blastoderm had increased, and the blasto- 

 pore was closed (fig. 64); its position (fig. 64 A), however, accurately located, but 

 more posterior than in the first stage; also the archenteron has greatly increased 

 in size. 



(c) At a third stage (fig. 65), the location of the blastopore can not be 

 accurately determined, although it is certainly near the hindmost point of the 

 blastoderm ; the archenteron is less definite, and its long axis, which remains parallel 

 to the neighboring germinal wall, becomes tilted backward, as indicated by the 

 arrow in the figure: and the cells, pm, which correspond to the ventral (posterior) 

 wall of the archenteron, now occupy a position further under and further forward 

 than in earlier stages, in consequence of the hindward extension of the blastoderm. 



(cf) Finally (fig. 66), this hindward extension is so expressed that the position 

 of the early blastopore shifts under the rim of the blastoderm and comes to appear 

 at the point bp; concomitantly the archenteron increases in size, its axis lying 

 nearly parallel to the surface and its ventral wall developing extensively both in 

 thickness and in (anterior) extension. From these conditions it follows that in the 

 later gastrulation of Chimsera we are dealing with a reopening of the blastopore of 

 an earlier stage. Accordingly, in contrast with gastrulation in sharks, Chimaera 

 preserves the primitive blastopore within the blastoderm itself. This stage, 

 however, is an evanescent one. In connection probably with a change in nutritive 

 values, whereby the yolk is passed to the archenteron from a source more and 

 more postero-ventral there is a constant tendency for the cells of the archenteron 

 to be drawn, both in ontogeny and in phylogeny, closer to the source of nutriment. 

 For this reason the cells of the archenteron multiply more rapidly from below than 

 from above (/'. e. , the region where primitively they were invaginated from the 

 ectoderm) with a result that the blastopore becomes of less and less importance in 

 early stages. It is suggested, also, that during this growth there is a constant 

 convection of the cells of the blastoderm, in the process of which elements formed 

 in the region of the posterior wall of the archenteron pass downward and forward. 

 Part passu, the posterior rim of the blastoderm, including the region of the blasto- 

 pore, extends first backward, then downward and inward; it thus comes finally to 

 lie under the rim (i. e., the later rim) of the blastoderm. 



