574 MORGAN AND HAZEN. [Vol. XVI. 



place at nearly the same rate from all points ; at least, until the 

 blastopore is much reduced in size. The outline of the blasto- 

 pore is generally oval, with the long axis connecting the dorsal 

 and ventral sides. At the time when the embryo is flattened 

 on the dorsal side to form the nerve plate, the blastopore 

 changes its shape, so that it becomes somewhat elongated from 

 side to side (PI. XXXIV, Figs. 13 and 14). 



Karyokinetic figures are found sometimes parallel to the 

 margin (PI. XXXIV, Fig. 12), sometimes at right angles to it. 

 There is great irregularity in the distribution of the dividing 

 cells. In some embryos a large number of cells are in process 

 of division, in other embryos nearly all the cells are in a resting 

 stage. Sometimes groups of cells may be found dividing and 

 others resting, but such occurrences are exceptional, and, as a 

 rule, the karyokinetic figures are scattered irregularly over the 

 surface. 



Lwoff has claimed that at a certain period cell division is 

 more rapid on the dorsal side of the blastopore than elsewhere, 

 and, in consequence, the ectoderm turns in and forms the dorsal 

 wall of the archenteron. A careful examination of a large num- 

 ber of embryos at all stages of development gives no support to 

 this view. 



It is important in comparing the changes in the shape of the 

 embryos of different stages to use the same series preserved 

 by the same methods. It is also important that sections in 

 the dorso-ventral plane and in oblique planes be carefully 

 distinguished. 



The early gastrula is shallow and saucer-shaped (PI. XXXIII, 

 Fig. 2). The lips are then brought nearer together, so that the 

 embryo is longer, but correspondingly narrower (PI. XXXIII, 

 Fig. 3). The embryo continues to narrow dorso-ventrally and 

 also laterally, and at the same time it grows longer. This 

 process continues, and for a time the growth of the dorsal 

 and ventral lips seem to be about equal. In still later stages 

 (PI. XXXIII, Fig. 5) the ventral wall seems to grow more 

 rapidly ; it bends inward toward the dorsal surface, and extends 

 farther posterior than does the dorsal lip. 



Since there are no definitely fixed points, it is very difficult 



