576 MORGAN AND HAZEN. [Vol. XVI. 



the ventral ; the ventral wall increasing very little, if at all, 

 during the period of closing. A comparison of our Figs. 2, 3, 

 and 5, PI. XXXIII, shows unmistakably that both the ventral 

 and dorsal walls are longer in the older stages. 



A comparison of Hatschek's Fig. 24 with our PI. XXXIII, 

 Fig. 2, will show that what Hatschek supposed to be the dorsal 

 side is in reality the ventral one. 



It is important to keep in view the different ways in which 

 the gastrulation might be interpreted, and we have tried to 

 illustrate in the accompanying text-figures the different possi- 

 bilities. The first series, I-V, represents a symmetrical closure 

 of the blastopore. The final closure at f is opposite the ante- 

 rior end c. The primary axis of the blastula and of the gastrula 

 corresponds with the antero-posterior axis of the embryo. The 

 closure of the blastopore is shown in V, and is seen to take 

 place equally from all points. Kowalevski supposed this to be 

 the way in which the blastopore closes. 



The second series, VI-X, represents an unsymmetrical clo- 

 sure of the blastopore. The ventral lip at a is supposed to be 

 fixed in VI, and in the successive figures, VII-IX, the dorsal 

 lip is represented as bending over and elongating to close the 

 blastopore. The closure of the blastopore takes place over the 

 dorsal side of the embryo. Fig. X shows the successive stages 

 in the closure of the blastopore. If we compare Figs. V and X, 

 we see that when the blastopore is turned upward toward the 

 observer the point of greatest bending of the wall is opposite 

 the blastopore in V, while in X the point of greatest bending 

 is anterior to the center of the blastopore (see also Fig, VIII). 

 This series, VI-X, represents Hatschek's idea as to the way in 

 which the blastopore closes. 



The reverse method of closing is shown in series XI-XV. 

 Here the dorsal lip is represented as fixed, and the ventral as 

 bending over to close the blastopore. Hence the closing is on 

 the ventral side of the embryo. 



It will also be noticed that in the last two series (VI-X and 

 XI-XV) the point of greatest bending shifts gradually around 

 the anterior end of the embryo as the closing takes place ; while 

 in the first series the point of greatest bending remains fixed. 



