MUSEUM OF COMPARATIVE ZOOLOGY. 87 



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

 shown in Figures 11 to 6, tliat is, to tlie iifth day ; but earlier than that 

 the conditions are much rliore obscure. 



In the earliest stages at which the difTerent 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 protovertebrte, 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 protovcrtebne are foirly 

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

 cells (i. c. VI.) between the lower margin of the protovcrtebne and the 

 lateral plate. -^ 



In later stages the flattening out of the fore-gut and the foi'mation of 

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

 accuracy ; but, if sections a little posterior to tliis 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 G, it is possible at 

 once to homologize the fore-gut, the lateral plates, the protovertcbra), 

 and the cells marked i. c. vi., with the structures correspondingly marked 

 in Figure 4. 



But if it be objected that a comparison of difterent 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 cori'cspond to one another throughout the 

 younger series. 



It follows, therefore, that the cells /. c. vi. in Figure 5 arc homologous 

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

 wi;,h the cells thus marked in Figure 2 may be also estal)lislied. 



If these colls are traced through the sei'ies of sections fi'om 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 tiiat, as the gut becomes flattened 



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

 that their proximal (median) niaririns are, like the eompressed vertebr;e, directed 

 downward (s<imewhat oblique^) toward the closed t'ore-irut, insteail of lyintr out 

 flat 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. 



