W. K. BEOOKS ON THE GENUS SALPA. 25 



the egg entirely fills the cavity of the follicle, as shown in Plate X, Fig. 1 ; 

 but as soon as segmentation begins, after fertilization, Fig. 3, the cavity 

 becomes divided into two portions, an empty one, 5, and another which is 

 occupied by the egg. Over the half occupied by the egg the follicle cells, 

 10, retain their original character and their sharp boundaries, while those 

 which form the wall of the empty half, 7, lose their distinctness, and 

 multiply rapidly by karyokinesis, so that this half soon becomes much 

 thicker than the other. For a short time the blastomeres, Fig. 5, 9, which 

 are formed by the segmentation of the egg, are bounded on one side by 

 the empty f ollicular cavity, o ; but the follicle cells soon begin to migrate 

 inwards in the zone which is marked 8 in Fig. 3, and soon completely 

 cover up the blastomeres, as shown in Fig. 5, 7 and 8, so that the folli- 

 cular cavity is now bounded on all sides by follicle cells, and the blasto- 

 meres are imbedded in a mass of follicle cells. The follicle cells in the area 

 which is marked 10 in Fig. 3, now begin to move inwards in radial lines 

 and to push their way in among the blastomeres, and to force these apart, 

 as is shown in Fig. 8, and this process of migration goes on until the folli- 

 cular cavity is obliterated, as shown in Fig. 9. At the stage shown in 

 Fig. 8, the embryo consists of the following structures : first, an outer or 

 somatic layer of follicle cells, 7; second, a visceral layer of follicle cells, 8; 

 third, an area, 10, where these two layers are continuous with each other ; 

 fourth, the blastomeres, 9 ; fifth, follicle cells between the scattered blas- 

 tomeres, and sixth, the follicular cavity, o. In a stage a little older, Fig. 

 9, and Plate XI, Figs. 1 and 2, the cavity of the follicle becomes obliterated 

 as already noted, and the follicle cells of the visceral layer begin to 

 multiply very rapidly by direct division of the nuclei, although the 

 somatic cells still multiply by karyokinesis, but much less actively. 



Plate XLII, Fig. 1, is a part of the embryo, shown in Plate XI, Fig. 

 2, very highly magnified. Three blastomeres, 9, and part of a fourth are 

 shown imbedded in the mass of follicle cells of the visceral layer, 8, which 

 is not separated by any empty space from the somatic layer, 7, of follicle 

 cells. The blastomeres are so much larger than the follicle cells that they 

 can be recognized without difficulty. 



Each of them has a very large spherical nucleus, with a complicated 

 network of very fine and delicate threads of chromatin. The nucleus is 

 near the center of the blastomere, which is itself nearly spherical, very 

 much more transparent than the surrounding follicle cells, with a well 

 marked boundary, and filled with a number of bodies which stain much 

 more deeply than the substance of the blastomere. The boundaries 



