McCrady.] 176 [April 18, 



itself as to cover their surfaces of contact with only a very thin layer, 

 while it masses itself on their free surfaces, and finally from the 

 accumulations thus formed again sends centripetally inwards the 

 division planes, which divide the first two segment spheres into four. 

 Kowalevsky remarks that the transparency of the egg during these 

 phases is remarkably favorable to the exhibition of a nucleus, if one 

 existed, yet he could find none. The impulse producing the seg- 

 mentation seems to proceed wholly from the outer layer of proto- 

 plasm (the ectoprotembryo) ; the inner or yolk responds to the 

 motions, only as a seemingly dead and motionless body responds to 

 mechanical pressure from without, possessing in itself no active in- 

 trinsic energy. 



15. In these cases of segmentation with an ectoprotembryo, the 

 theory here presented regards the segmentation as effected by the 

 amoeboid activity of its protoplasm operating from without inwards. 

 There is obviously only a difference of degree and circumstance be- 

 tween the putting forth of pseudopodia (which in many Amoeba? are 

 merely large lobes) and the inward extension of such plates of proto- 

 plasm as those here effecting the division of the yolk. When two 

 lobe-like pseudopods are developed in any Amoeba, an interval of 

 emargination or depression is necessarily constituted between two 

 pseudopodial lobes. If the lobes enlarge still further, they tend to 

 engross the whole protoplasm of the animal, and to divide the food- 

 mass contained iu it between them. This is tantamount to an in- 

 crease of the depression to a furrow, of the furrow to a cleavage 

 plane ; and a comparatively slight increase of such a furrow might in 

 any case divide an Amoeba into two. This actually happens when, 

 as is sometimes witnessed, a large pseudopod actually detaches itself 

 from the main body and becomes free. 



16. In a large number of segnientational developments, however, 

 the protembryo is massed centrally] thus constituting the " embryonal 

 vesicle " or " nucleus "or " persistent germinative vesicle ' ' of vari- 

 ous writers. In these cases the segmentational influence seems to 

 proceed from within outwards. The entoprotembryo being located 

 at some point within the yolk mass, must indeed necessarily act cen- 

 trifugally in possessing itself of the yolk. Under the provisional 

 theory here explained, its mode of effecting this will still be amoe- 

 boid ; i. e., the entoprotembryo will put out pseudopodia radially 

 extending through the yolk mass towards the surface of the egg. As 

 such pseudopodia are known to form net works and detached masses 



