100 



J. Thos. Patterson. 



side one finds the posterior margin becoming less thick and gradu- 

 ally taking on the syncytial condition characteristic of the anterior 

 and lateral parts of the zone of junction. However, it is not until 

 one has reached about 45 degrees to either side of the median line 

 that the posterior margin is found to be reduced to the average thick- 

 ness of the rest of the edge. 



Closing of the Blastopore. — It was stated above that the entoderm 

 grows forward through the subgerminal cavity. The source from 



Fig. XIII. Reconstruction of a blastoderm taken thirty-five and one- 

 fourth hours after fertilization. The blastopore has just closed, and the 

 zone of junction completely encircles the blastoderm. D, is the enclosed 

 dorsal lip. vv', w\v', and xx' are the planes of the sections illustrated in 

 Figs. 24, 23, and 22 respectively, x 27.2. 



which it draws its material for this forward growth is, of course, 

 the thick dorsal lip. This results in producing either one of two 

 conditions in the lip at the time the blastopore is closed. On the 

 one hand, the entoderm may have grown forward to such an extent 

 as to have produced a very perceptible diminution in the lip before 

 the closing occurs. Such is the case in the blastoderm represented 

 in Fig. XII, as is apparent from the median section (Figs. 20 and 



