No. 2.] ORGANIZATION OF THE EGG OF UNIO. 265 



the anterior end of the ovum, near the posterior pole a second 

 circle of three, "while the eighth cell D occupies an almost 

 polar position." In the undivided egg there is a central nucle- 

 ated mass of protoplasm connected by strands with a periph- 

 eral layer. Divisions begin in this central mass and do not 

 affect the yolk, so the cells are united by protoplasm, although 

 they move apart. This lack of contact of the cells character- 

 izes the early cleavage throughout, so that, as McMurrich points 

 out, none of the laws of alternating cleavages or mutual pres- 

 sure can influence the form of cleavage. Yet the cleavage 

 does follow an invariable law of its own, and the arrangement 

 described for the 8-cell stage is brought about partly by the 

 orientation of the spindles and partly by extensive migrations 

 of cells. Now it is to be observed that this arrangement is an 

 adaptive one in the sense that it predelineates definite systems 

 of organs. The four anterior cells give rise to the ectoderm, 

 the three next to entoderm and some mesoderm, while the pos- 

 terior cell forms the vitillophags, " still later giving rise to cer- 

 tain mesodermal structures." 



" In the later stages oi Jaera there is a concentration of cells 

 towards one surface of the ovum, which will eventually become 

 the ventral surface of the embryo ; concomitantly with this con- 

 centration the outlines of the naupliar region of the embryo 

 being formed." But in Porcellio, "at the period at which the 

 concentration of the peripheral protoplasm occurs," to form the 

 ventral region of the embryo, " the nuclei are separated from it 

 by equal and considerable distances, being united with it, how- 

 ever, by the protoplasmic network, and it is difficult to conceive 

 how any of them could be able to influence the peripheral cyto- 

 plasm in such a way as to produce the concentration. It seems 

 rather that we have to do with an independent action of the 

 cytoplasm, which precociously prepares for the formation of the 

 blastoderm." 



Have we not here a beautiful picture of the way in which 

 the egg acts as an undivided organism, controlling some events 

 by orientation of cleavage planes or shifting of cells, and others 

 by protoplasmic movements clearly independent of cell bound- 

 aries } Is it not evident that the orientation of the cytoplasm 



