142 



BIOLOGICAL LECTURES. 



which will eventually become the ventral surface of the em- 

 bryo, concomitantly with this concentration the outlines of the 

 naupliar region of the embryo being formed. In Porcellio and 

 Armadillidmm a similar concentration occurs, but in these 

 forms the development of the naupliar region of the embryo is 

 retarded, and one finds at an early stage a layer of cells closely 

 aggregated together at one portion of the surface of the ovum, 

 a few scattered cells being distributed over the rest of it. 

 This aggregation of cells may be termed the blastoderm. 



To return now to the phenomenon to which I wish to call 

 attention. At the conclusion of the second division of the 

 nucleus in Porcellio, one finds that the peripheral protoplasm is 

 no longer uniformly distributed over the surface of the ovum, 

 but there has been a concentration of a large amount of it to 



one portion of the surface (Fig. 10). 

 The nuclei are still imbedded in the 

 yolk, and only after several divi- 

 sions do they complete their cen- 

 trifugal migration, entering the 

 peripheral protoplasm and forming 

 with it the blastoderm. At the 

 period at which the concentration 

 of the peripheral protoplasm occurs 

 the nuclei are separated from it by 

 equal and considerable distances, 

 being united with it, however, by 

 the protoplasmic network, and it is 

 difficult to perceive how any of them could be able to influence 

 the peripheral cytoplasm in such a way as to produce the con- 

 centration. It seems rather that we have to do with an 

 independent action of the cytoplasm, which precociously pre- 

 pares for the formation of the blastoderm. 



We are now, I believe, in a position to appreciate the signif- 

 icance of the peculiar direction of the spindle of the nucleus D 

 of the ovum oijaera, a significance which is indicated by what 

 has been said regarding the aggregation of the peripheral pro- 

 toplasm of Porcellio. Both phenomena are simply precocious 

 preparations for a differentiation which will later become pro- 



FlG. 10. 



