390 C. 0. WHITMAN 



absurdity. Now, there is no difference of opinion as to the 

 fact that the final closing in of the yolk takes place " at some 

 little distance behind the embryo " in the case of the Elasmo- 

 branch. A glance at Balfour's fig. 30 b (' Corapar. Embryol./ 

 p. 52) will show the extent of the line of concrescence. It 

 will be seen, at the utmost, this line can only be said to 

 extend from the region of the head along the dorsal side to the 

 tip of the tail, from this point forward on the ventral side 

 to the umbilical stalk, and from this stalk backward along 

 *' the linear streak " which connects the embryo with the edge 

 of the blastoderm. The closure of the space {ijk) will com- 

 plete the line of concrescence. It will be seen that there is 

 an important ventral portion of the body, extending from the 

 umbilical cord to the head, Avhich is in no sense of the word 

 formed by concrescence. If we take into consideration the 

 entire yolk, as Balfour certainly does, it is plain that the line 

 of concrescence is considerably less than half of the entire cir- 

 cumference. When we remember that among our invertebrate 

 vermian relatives the cases are not at all rare in which this 

 line is much more than half the circumference of the eg^, we 

 must admit that nature delights in just such absurdities as 

 Balfour has pointed out. 



The fifth objection presents the same difficulty that we meet 

 with in the development of the Bird, where, owing to the pre- 

 sence of an enormous quantity of food-yolk, the process of con- 

 crescence has undergone such extreme modification that the 

 only constant outward manifestation of it is the primitive 

 groove- But the connection of the primitive streak with the 

 area opaca, the relations of thickness between different parts 

 of the blastoderm at successive stages, the occasional appear- 

 ance of a marginal notch, the extension of the primitive 

 groove to the notch in rare cases, the occurrence of a neuren- 

 teric canal, and other considerations of a comparative embryo- 

 logical nature, appear to me to outweigh the objection, and 

 compel us to recognise here the same principle of embryonic 

 formation that characterises the more primitive forms of the 

 vertebrate stock. 



