lyo CONKLIN. [Vol. XIII. 



Blochmann finds the cause of this forward movement of the 

 ectoblast in the formation of an ectodermic invagination at the 

 apex. Apart from the doubtful character of this invagination, 

 to which I have already referred, there is such a mass of evi- 

 dence in favor of the view that the forward movement is due to 

 the greater growth at the posterior pole that I think the cause 

 assigned by Blochmann may fairly be called in question. 



In some gasteropods, in which there is a small quantity of 

 yolk, the change of axis occurs in the method typical for the 

 trochophore, vis., the apical pole does not change position, but 

 the oral pole is shoved forward on the ventral side, probably 

 by the development of a structure homologous with the ventral 

 plate of the annelids. Such a process has been described by 

 Patten ('86) in the case of Patella. 



These shiftings of the embryonic axes are apparently charac- 

 teristic of all Mollusca, and they present no essential difference 

 from the methods which Alex. Goette ('82) has shown are 

 characteristic also of turbellarians, nemertians, and annelids. 



The question of the origin of bilateral from radial forms, and 

 the consequent establishment of the antero-posterior axis among 

 all bilateralia is one which has received much attention from 

 some of the most eminent men of science. 



Lankester ('77) long ago suggested that the antero-posterior 

 axis of bilateral animals corresponds to the chief axis of coelen- 

 terates, the oral pole of the radiate forming the anterior pole of 

 the bilateral animal, and the aboral part of the coelenterate 

 lengthening to form the trunk of the bilateral animal. 



Balfour, on the other hand, held that "the conversion of 

 such a radiate form into the bilateral took place not by the 

 elongation of the aboral surface and the formation of an anus 

 there, but by the unequal elongation of the oral face, . . . while 

 the aboral surface became the dorsal surface." This conclu- 

 sion was elaborated by Balfour, and after him by Sedgwick. 

 Both supposed the central nervous system of arthropods, anne- 

 lids, mollusks, and chordates to be derived from a circumoral 

 nerve ring. 



After proposing this theory, Balfour says {Comp. Embryology, 

 vol. II, p. 379): "The position of the flagellum in Pilidium 



