5 86 MORGAN AND HAZEN [Vol. XVI. 



deeper on the ventral side, making the outUne of the embryo 

 more rounded there. A somewhat similar effect is sometimes 

 produced by an oblique section through a slightly elongated 

 embryo. Sections similar to text-fig. XXIX d are some- 

 times obtained, but it is unsafe to judge from such a section 

 alone that the more rounded side is the ventral one. The 

 entire series of sections must be known, a-f. During the 

 first stages of gastrulation there is an extraordinary amount of 

 variation in the shape of the embryos. Forms with a some- 

 what elongated blastopore are not infrequent, and an oblique 

 section of such an -embryo produces the effect shown in the 

 figures. Even in the later stages, the same asymmetry may 

 be found as shown in PI. XXXIII, Fig. ^ A. As a matter of 

 fact, the dorsal side of the embryo is in this case the shorter 



Fig. XXIX. 



one, but other embryos of the same age may be cut in such a 

 way that the shorter side is the ventral one. It would there- 

 fore be easy to arrange a series of these forms in such a way 

 as to make it appear that the closure of the blastopore is over 

 the dorsal side. Only after a careful and prolonged examina- 

 tion of both sections and surface views have we been convinced 

 that this condition is the result of an oblique section of a some- 

 what elongated embryo. If, for instance, the series of sections 

 to which XXIX c belongs, be examined, it will be found that 

 at one end of the embryo the sections are completely closed, 

 a-b ; but at the other end of the series the opening of the 

 blastopore enlarges, as shown in e,f. 



Sections of the kind, with the dorsal side shorter than the 

 ventral, might seem to confirm Hatschek's view of the closing 

 of the blastopore over the dorsal side, but we hope we have 

 shown this view to be erroneous. Without other means of 

 orientation the shape of the section may be very misleading, 



