1877.] 181 [McCrady. 



the yolk and thus forms a thin stratum enveloping the protoplasm of 

 the yolk. It seems to me that the observation of Hertwig should be 

 interpreted in the same way, though he prefers the notion that the 

 material of this vesicle is finally absorbed by the yolk; meaning, I 

 presume, by absorption, assimilation, which would involve the com- 

 plete withdrawal of the germinative vesicle from all participation in 

 the constitution of the new animal or protembryo. Butschli indeed 

 suggests a mixing of the matter of the germinative vesicle with the 

 superficial protoplasm of the yolk ; but he gives an alternative which 

 is more consistent with our provisional theory when he speaks of it 

 as possibly an " indistinguishable on- and inlaying " of these proto- 

 plasms together. Besides, Butschli regards the nucleus of segmenta- 

 tion as formed out of the earlier material of the germinative vesicle, 

 a portion of which forms a prolongation penetrating inwards into the 

 yolk which surrounds it. This portion, then, according to his view, 

 becomes detached from the superficial protoplasm of the germinative 

 vesicle and forms the nucleus; which, subsequently, is integrated with 

 that other problematic body, regarded by Auerbach and Butschli as 

 a second nucleus, and by Hertwig as a probable spermatozoon head or 

 nucleus. Butschli seems to suppose the spermatozoon to be arrested 

 at the surface, and then to be united with the peripherally disposed 

 germinative vesicle, from which subsequently two or more nuclei are 

 formed, and then uniting, compose the first nucleus of segmentation. 

 But Hertwig's observation in one case of a linear appendage to this 

 penetrating body, gives a great air of probability to his interpretation 

 of it as a spermatozoon, or part of a spermatozoon. 



If the peripheral disposition of the protoplasm of the germinal 

 vesicle essentially corresponds with the conception of my ectoprotem- 

 bryo, the radial extensions from the nucleus, observed by all these 

 embryologists, seem to be identical with the radial pseudoped-like 

 extensions assumed in this theory as put forth by the entoprotembryo. 

 There is, however, nothing yet observed corresponding to the in- 

 wardly directed pseudopodial extensions which the theory assumes to 

 be the agency by which the ectoprotembryo appropriates the yolk. 

 We can hardly doubt, however, that in the case of many insects and 

 Crustacea, there is an actual ectoprotembryonic disposition of the 

 germ, and consequently an appropriation of the yolk from without, 

 such as I have sought to explain by inwardly directed protoplasmic 

 processes of a temporary character. 



On the other hand, and in contrast to these correspondences, there 



