SOME PROBLEMS OF ANNELID MORPHOLOGY. 



73 



e^AW 



.NNOVVW. 



the ventral surface of the embryo (Figs. 5, 6), and that 

 the elongation of the body takes place approximately 

 in the plane of the blastopore — i.e. that the long axis 

 of the adult coincides with one of the transverse axes of 

 the gastrula. This appears with especial clearness in 

 the development of annelids (Fig. 5), 

 where the bilaterality is thrown back, 

 in a measure, upon the gastrula itself, 

 and the blastopore is more or less 

 elongated, its anterior part persisting 

 as the mouth ; or in that most primitive 

 arthropod, Peripatus, where the elon- 

 gated blastopore closes in the middle, 

 the two openings thus left persisting 

 as mouth and anus respectively (Fig. 6). 

 In these facts lies, as I believe, the 

 key to the problem of concrescence. We 

 see from such cases as the earthworm 

 and Peiipatus, that the separation of 

 the two sides of the body (germ-bands), 

 may be caused, not by a mass of food- 

 yolk, but by the blastopore itself, and 

 concrescence is a sequence of the closure of 

 the blastopore, modified more or less ex- 

 tensively by accumulation of yolk. In 

 forms like the leeches or vertebrates 

 concrescence is modified and exaggerated by the fact 

 that the region of the blastopore is occupied by the 

 enormous mass of yolk. But this should not blind us to 

 the fact that the primary cause of concrescence lies in 

 the position of the blastopore, not of the food-yolk. 

 From a genealogical point of view I believe this must be 



Figs. 5 and 6. 



