No. I.] 



GAR-PIKE AND STURGEON. 



47 



Acipenser presents a suggestive contrast with that of Lepi- 

 dosteus (p. 24). 



Middle layer 

 appears 



Its early char- 

 acter 



Its mode of 

 growth 



Lepidosteus. 



Late : about the time of the 

 closure of the blastopore. 



A discoidal cell mass, almost 

 symmetrical, its elements 

 alone in contact with inner 

 and outer germ layers at its 

 center, the blastopore. 



At first peristomal ; becomes 

 greatly thinned : at its per- 

 iphery single-celled, mesen- 

 chymatous. Gastral meso- 

 derm early apparent in 

 hinder region of embryo's 

 axis thickened notably near 

 the median plane (in stages 

 of PI. I, Figs. 18, 19), con- 

 nected with entoderm in the 

 hinder region of the embryo's 

 axis. 



Acipenser. 



Early : shortly after the appear- 

 ance of the ventral lip of the 

 blastopore. 



A ring-like cell mass, notably 

 asymmetrical, confluent with 

 inner and outer layers at the 

 rim of the blastopore, and 

 confluent at its outer (pe- 

 ripheral) margin with the 

 yolk mass. 



Notably peristomal, a layer of 

 almost uniform thickness 

 from rim of blastopore to 

 undifferentiated yolk tissue. 

 At stage of PI. IV, Fig. 66, 

 its thickness has become re- 

 duced in its peripheral region. 

 Gastral mesoblast differs lit- 

 tle in thickness from neigh- 

 boring peristomal mesoblast: 

 the region of its connection 

 with entoderm is restricted 

 to that of the hinder one- 

 sixth (about) of the embryo's 

 axis. 



The earlier appearance of the mesoblast in Acipenser seems 

 to the writer to be probably due to the smaller amount of food 

 yolk in this form. The difference, however, in the plane of 

 early mesoblastic growth is hardly to be ascribed to this cause 

 alone : the extremely compressed or rather horizontal mode of 

 growth of the Sturgeon would, however, seem adequate to 

 explain this character. The mesoblast has a normal separate 

 growth till in the region of the end of the coelenteron : a pos- 

 sible mechanical cause may here prevent its separate extension ; 

 and a similar reason would account for the uniform thickness 

 of the layer of gastral mesoblast. 



