402 T. H. MOEGAN AND UME TSUDA. 



than in the last case, so that not only the anterior connection, 

 but also the anterior end of the medullary folds, lie on the 

 black portion of the egg. This embryo shows conclusively 

 that the extent of closure of the blastopore is far more than 

 the normal, for if the embryo really had covered so much of 

 the sphere as the whole of the white area, and as much of the 

 black as the anterior end now occupies, it would have covered 

 nearly two thirds of the sphere, and not one third, as in the 

 normal embryo. 



Fig. XIX is from an embryo in which the dorsal lip of the 

 blastopore has grown over the lower pole to the extent indi- 

 cated by the medullary folds. When looked at from below — 

 i. e. with the white area turned up — we see still a large ex- 

 posure of white, but the posterior extension of the blastopore 

 is not completed, again verifying the statement made above, 

 that the exposure of the white is in these eggs abnormally 

 extensive. 



In fig. XX is drawn the posterior end of an embryo much 

 more advanced than the last. Quite a large exposure of yolk 

 is present, but not nearly so much as in the other cases. 

 Anterior to the blastoporic plug the medullary folds have met 

 to form a closed tube. Posterior to the blastopore, as seen in 

 the figure, a deep groove is present, and this groove is formed 

 by the posterior medullary folds. The ventral lip of the blas- 

 topore has, therefore, grown over the white to the extent indi- 

 cated by the medullary folds. Whether this forward growth of 

 the ventral lip is unusually extensive I do not know, nor have 

 I any records to show whether in the earlier stages so much 

 of the white was enclosed as in the preceding cases; but, 

 judging';from the length of the embryo and from other facts, I 

 think we may safely conclude that the area enclosed was less. 

 Other embryos, with still less exposure of white, need not 

 be figured at present; Hertwig's description seems to cover 

 such cases. 



Serial sections were made through these embryos. In the 

 embryo shown in figs, xiv, xvi, &c., sections add little know- 

 ledge to that formed from surface views. The most noticeable 



