MUSEUM OF COMPARATIVE ZOOLOGY. 87 



the cells in question (i. c. m.) backward in time through the stages 

 shown in Figures 11 to 6, that is, to the fifth day ; but earlier than that 

 the conditions are much more obscure. 



In the earliest stages at which the different elements can be distin- 

 guished in our sections, — about the third day (Fig. 2), — the fore-gut 

 can be seen partly formed, the lateral plates, the protovertebrae, and a 

 few scattered cells (i. c. m.) between the lower part of the protovertebrse 

 and the lateral plates. In the next stage (Fig. 4), the fore-gut is com- 

 pletely closed, and both the lateral plates and pro to vertebrae are fairly 

 well defined. There is now a considerably larger number of scattered 

 cells (i. c. m.) between the lower margin of the protovertebrse and the 

 lateral plate. -^ 



In later stages the flattening out of the fore-gut and the formation of 

 the gill slits in this region make it difficult to homologize the parts with 

 accuracy ; but, if sections a little posterior to this region are examined, 

 a condition is found that is perfectly comparable with the earlier stages 

 in this region. Taking, for instance. Figure 5, which represents a sec- 

 tion 0.09 mm. posterior to the one shown in Figure 6, it is possible at 

 once to homologize the fore-gut, the lateral plates, the protovertebrse, 

 and the cells marked ^. c. m., with the structures correspondingly marked 

 in Figure 4. 



But if it be objected that a comparison of different regions proves 

 nothing, one may begin by comparing with Figure 5 sections in the 

 younger of the two stages, 0.09 mm. posterior to that of Figure 4. The 

 homology of the parts is in this instance perfectly clear ; the correspond- 

 ing parts may then be traced in this younger series forward to the sec- 

 tion shown in Figure 4, It will be found that there is no difficulty in 

 identifying the parts which correspond to one another throughout the 

 younger series. 



It follows, therefore, that the cells i. c. m. in Figure 5 are homologous 

 with those so marked in Figure 4, and by a similar method their identity 

 with the cells thus marked in Figure 2 may be also established. 



If these cells are traced through the series of sections from which both 

 Figures 5 and 6 are taken, the result is as follows. In passing forward 

 from the region of Figure 5, it is seen that, as the gut becomes flattened 



1 The lateral plates are so folded by the sinking of the embryo into tlie yolk, 

 that their proximal (median) margins are, like the compressed vertebra, directed 

 downward (somewhat obliquely) toward the closed fore-gut, instead of lyln<r out 

 Hat on the surface of the yolk, as occurs in the case of the embryos of fishes which 

 are not so deeply imbedded in the yolk. 



