92 BULLETIN OF THE 



of fact they are depressed (PI. V. fig. 28 ; PI. VI. fig. 30), and, there- 

 fore, can be accounted for more reasonably on the supposition of a centri- 

 petal force. It has already been indicated that these areas are formed 

 during the contraction of the vitellus ; surface views and sections com- 

 bined show that the very plastic protoplasm of the blastema is moulded 

 to the surface of the peripheral yolk corpuscles, to which it at fir.st 

 forms caps, all of the caps being joined at their margins. Owing to mu- 

 tual pressure these subsequently appear as polygonal areas. The cause, 

 then, producing this surface phenomenon is mechanical, and depends 

 upon the contraction of the protoplasm of the egg. But under what 

 influence does this contraction transpire 1 As the phenomenon takes 

 place during the stage which is characterized by the existence of the first 

 segmentation-nucleus, it is more than probable that the cause if. resident 

 in this central nucleus, which exerts its attractive influence on all the 

 protoplasm of the egg, but finds its external manifestation at this period 

 principally in the blastema. Thus, it is probable that the same cause 

 which produces in these eggs contraction of the vitellus, also induces the 

 division of the blastema into areas. 



The maimer in which the protoplasm acts upon the yolk in the assimi- 

 lation of its substance is clearly indicated in the blastema stage. That 

 portion of the surface of the corpuscles which is in contact with the pro- 

 toplasm of the blastema appears deeply eroded (PL VI. figs. 30-33), and 

 the fine fragments into which the detached yolk substance is broken 

 gradually merge into the still more finely granular protoplasm. A some- 

 what similar fate overtakes the yolk corpuscles in the vicinity of the 

 central nuclei (PI. V. figs. 28, 29; PI. VI. fig. 34). Here, however, 

 there is a very gradual transition from the larger corpuscles to the much 

 smaller ones which immediately surround the finely granular protoplasm 

 of these central cells, — a process of fragmentation appears to precede the 

 erosion, and thereby a much greater surface of yolk substance is exposed 

 to the action of the protoplasm. In the former case there is no total 

 fragmentation of the yolk corpuscles, and the erosion proceeds from one 

 side only, leaving the opposite side with a sharp, more or less even out- 

 line. The preliminary fragmentation of the yolk in the vicinity of the 

 rapidly proliferating cells is, without question, correlated with the rapid 

 growth of the latter. 



ClaparMe ('62), Barrois (78), and Balfour ('80), have each given 

 explanations of the reversion of the ewhryo. Balfour's is the simplest. 



