572 MORGAN AND HAZEN. [Vol. XVI. 



closely the cells of the outer surface of the dorsal side 

 (PI. XXXIII, Fig. i). On the other hand, at the ventral lip 

 there is rather an abrupt transition between outer and inner 

 cells (PI. XXXIII, Fig. 2). MacBride has called attention to 

 these differences, but it is not improbable that in the early 

 stages, at least, he has confused the dorsal and the ventral 

 sides of the gastrula. 



Although the embryos can be oriented, as stated above, yet 

 it is still very difificult to determine how the closure of the 

 blastopore is brought about. While the blastopore becomes 

 smaller, the embryo is, at the same time, changing its shape, 

 so that it is not possible to assume that any one point is fixed 

 in relation to the others. Moreover, the possibility of cell- 

 migration must also be kept in mind. Lwoff has considered 

 the presence of karyokinetic division as a criterion of growth ; 

 but it must not be forgotten that growth need not follow 

 unless after division the cells increase in size. Furthermore, 

 cell migration is known to take place without cell division.^ 



As stated above, we had hoped to make use of the vegetative 

 pore, and by this means to determine how the closure of the 

 blastopore takes place. The pore is most often present in 

 series of eggs that do not develop normally, and hence there 

 is a certain amount of risk in using such a feature to deter- 

 mine the changes that take place in the normal ^^g. The 

 pore seems generally to disappear in the later stages. More- 

 over, other openings are sometimes found in the endoderm, due, 

 in some cases at least, to changes in shape of the cells at the 

 time of division. PI. XXXIV, Fig. 17, shows a cross-section 

 of an Q.gg in which the vegetative pore is large, and lies near 

 the highest point of the invaginated cells. 



Occasionally a gastrula is found with the cells around the 

 vegetative pore turned outward (PI. XXXIV, Fig. 16); and the 

 cells remain in this position even during later stages, as shown 

 in PI. XXXIV, Fig. 19. This embryo throws some light on 

 the way in which the blastopore closes (PI. XXXIV, Fig. 19). 

 As it was found among embryos in which, on an average, the 



1 In the sea-urchin the archenteron is formed by cells pushing into the seg- 

 mentation cavity. 



