^jS MORGAN AND HAZEN. [Vol. XVI. 



dorsal side in XVIII ; and, third, the more rapid growth of the 

 ventral wall in the final stage of closure. 



Cell Division during Gastriilation. 



Cell division occurs throughout the entire period of gastru- 

 lation both in the ectoderm and in the endoderm. We have 

 examined a large number of preparations, both sections and 

 surface views, to see whether cell division is more frequent in 

 one part of the embryo than in another. Although cell division 

 is present at all times and in all stages, yet we have found, in 

 general, no region of more rapid cell multiplication. 



Great individual differences exist, for while one preparation 

 may show more karyokinetic figures on the dorsal or on the 

 ventral lip of the blastopore, other preparations of the same 

 age may show other regions of cell multiplication. Hence, 

 with a preconceived idea as to the place at which more rapid 

 growth occurs, one could easily find preparations confirming 

 such a view. But an unprejudiced examination shows that 

 cell division is general and not restricted to any special region. 

 Text-figs. XXI-XXVIII show in what regions cell division is 

 taking place in certain embryos of various ages. The drawings 

 are diagrammatic reconstructions of serial sections. The dorsal 

 side is indicated by the parallel lines. One figure of each pair 

 shows, therefore, the dorsal, and the other the ventral side. 

 The dividing nuclei in the ectoderm are represented by solid 

 dots, and those in the endoderm by circles. 



Fig. XXI a, b represents the embryo at midnight, corre- 

 sponding in age with the embryo shown in PI. XXXIII, Fig. 2. 

 Only two ectodermal cells are dividing, while many endodermal 

 cells are in process of division. Another embryo of the same 

 age is shown in Fig. XXII a, b. Here the number of cells 

 dividing in ectoderm and endoderm is about the same. The 

 dividing cells are scattered nearly equally throughout the 

 embryo. 



The next stage, Figs. XXIII and XXIV a, b, represent the 

 embryo at 3.30 a.m. Both show cells dividing around the blas- 

 topore, and there is also a region of division at the anterior end. 



