FATE OF BLASTOPORE, ETC., IX CHELOXIA. H^ 



towards the periphery of the shield." For details, the reader is 

 referred to that paper. 



I now come 1(3 that part of Will's paper which at present 

 concerns ns most — viz., to that part which treats of the formation and 

 nature of the primitive streak and groove. I shall not here go into 

 the theory which he advances and illustrates by the figure L (p. 61), 

 for at tlie present state of our knowledge a theory that makes the 

 primitive streak grow forwards from the posterior end is in my opinion 

 not tenable. And if I understand Will aright, his theory has such a 

 characteristic. Duval's observations alone would be enough to 

 contradict such a theory, not to speak of many facts which have 

 been brought forward in support of the concrescence-theory. I shall 

 therefore here rather confine myself to the account which Will gives 

 of the chans'es observable on the surface and in sections. And in this 

 I confess to a sense of disappointment. A glance over his figures 

 shows that he had before him some significant stages very much 

 resembling those I give in this paper. I refer more especially to those 

 on his Taf. 3. But the explanation he gives of their nature seems to 

 me to be wholly inadequate. He simply restates his views, just as I 

 have given them in the earlier part of the foregoing article (pj). 8—10), 

 and explains everything by them. Thus, in his Figs. 22-27, in which 

 a narrow groove goes backwards from the posterior end of the 

 medullary groove — very much like my Figs. 4 et seq. of the foregoing 

 article, if we take away the yolk-plug at the posterior end of the 

 primitive groove — he considers the primitive groove produced by the 

 simple coming together of the blastopore lips, after tlie sinhin<j below of 

 the yolk-plug. Moreover, he considers that the primitive streak is 

 posteriorly entirely surrounded by the medullary folds (ji. 70), and 

 therefore all the changes, he describes, in the primitive streak takes 

 place presumably within the medullary folds. He has tigere fore evtirelij 



