DEVELOPMENT OF A CH1M.KROID. 



277 



of a large part of the entire egg, a process unique among verte- 

 brates, and suggesting only remotely a physiological parallelism 

 with developmental features in certain lower forms, e.g., digenetic 

 trematodes. Thus in a stage of late blastula the surface view of 

 the egg (Fig. 13), shows with the unaided eye a distinct fissure 





FIG. 1 1 A. Detail of the region of the blastopore of Fig. n. 



passing near the side of the egg, between the points * and *, and 

 into this run smaller ones. 



That this is a fissure and not merely a surface marking becomes 

 clear when the egg is hardened (e.g., in corrosive-acetic), for the 

 sides of the fissure then separate so that one can see between 



fe^ BP 



- ' usifb 







SO 



FIG. 12. Later gastrula. Lettering as in foregoing figure. 



them deep into the yolk. In later stages (Figs. 14 and 15) 

 similar conditions maintain, the fissures in these cases, however, 

 extending outward from near the germinal region. In these 

 cases it is found that the ventral side of the egg becomes more 

 completely fragmented than the upper ; thus the ventral side of 

 the stage of Fig. 15 shows a variety of surface furrows (Fig. 16), 



