OP ARTS AND SCIENCES. 49 



the succeeding amphiastral phase. At every clearing up we observe 

 that the bhistodisc is increasing in size, while the periblast becomes 

 thinner and thinner. There can be no doubt about the fact that the 

 blastodisc actually draws the larger portion of the periblast into itself 

 during the first four cleavage-stages. 



We have seen that each cleavage-stage comprises two distinct acts : 

 first, an expansion which accompanies the elongation and division of 

 the amphiasters ; and second, a contraction or concentr-ation of the 

 protoplasm around the new generation of nuclei. At each succeeding 

 contraction, or systole as it might be called, a portion of the jjeriblast 

 becomes incorporated into the blastodisc ; and the outcome of this is, 

 that the latter thickens up and soon presents the form of a calotte. 

 The question naturally arises, What determines these relations between 

 the blastodisc and the periblast? It is evident that there is some 

 attractive force in the blastodisc which is absent from the periblast ; 

 and further, that this force, whatever it may be, increases during the 

 cleavage. Now the nucleo-plasm is the only substance, so far as we 

 can ascertain, which is contained in the blastodisc and not contained 

 in the periblast; and it is a very interesting fact that this nucleo- 

 plasm increases at a much more rapid rate than the blastodisc. We 

 cannot here enter into a discussion of the nature of the nuclei ; but the 

 conclusion is unavoidable that the attractive power of the blastodisc 

 resides either (1.) in the nuclei ; or (2.) in a special portion of the pro- 

 toplasm intimately associated with the nuclei in the jirocess of division ; 

 or (3.) in both. So far as the question we are here considering is 

 concerned, it is a matter of indifference which hypothesis we adopt. 



The cleavage process not only increases the number and volume of 

 the nuclei, but it distributes them throughout the blastodisc. It is the 

 establishment of these centres of attraction in the margin of the 

 blastodisc, which accounts for the transformation of the blastodisc into 

 a cap-like body. Pari passu with the multiplication of these centres, 

 the blastodisc rises above the niveau of the egg-sphere. Its margin 

 thickens up at the expense of the periblast, becomes steep at the end 

 of the first cleavage, thicker and slightly rounded at the end of the 

 second, more strongly rounded (forming a re-entrant angle with the 

 periblast) at the conclusion of the third, and deeply constricted from 

 the egg-sphere during the 1 6-cell stage. The progressive deepening 

 of the re-entrant angle carries the zone of junction a short distance 

 under the margin of the blastodisc, giving thus the appearance of a 

 complete separation from the periblast. But the continuity is still 

 preserved, and is destined to remain for some time to come. 



VOL. XX. (n. S. XII.) 4 



