W. K. BROOKS ON THE GENUS SALPA. 263 



The cells of the visceral layer not only cover up the blastomeres, but 

 soon push in between them and separate them from each other, as shown 

 in Plate X, Fig. 8, in such a way as to form a solid mass of cells, which 

 we may well call, with Salensky, the embryonic mass. It is made up, as 

 Salensky has shown, of blastomeres and migratory follicle cells in the 

 most intimate relation to each other. 



We cannot follow by sections the path which each migratory cell 

 takes, but it seems probable to me that those follicle cells which push in 

 between the blastomeres come from the area marked 10 in Plate X, 

 Fig. 8, where the somatic and splanchnic layers of the follicle are con- 

 tinuous with each other, while the cells which cover up the blastomeres 

 push in around the zone which is marked 8 in Figs. 3 and 5, Plate X. 

 The persistency of the cell boundaries in the area 10 may be due to the 

 fact that the plane in which they move during migration is perpen- 

 dicular to the surface of the follicle, while the proliferating cells of the 

 somatic layer of the follicle, 7, must move along parallel to the surface 

 for a considerable distance before they reach the zone where migration 

 takes place. 



The embryonic mass grows by the addition of new follicle cells until 

 it completely fills up and obliterates the follicular cavity, as is shown in 

 Plate X, Fig. 9, and Plate XI, Figs. 1 and 2, where the somatic layer, 7, 

 of the follicle and the splanchnic layer are in contact with each other. 

 While the change is taking place the migration of cells from the somatic 

 layer gradually comes to an end everywhere except within the area 10, 

 where the two layers are continuous with each other. The somatic cells 

 gradually become polygonal, their walls clearly defined, and they thus 

 come to resemble those which make up the whole follicle before fertili- 

 zation, as will be seen from a comparison of Plate X, Fig. 1, with Fig. 9. 

 In this last figure the area, 10, where the two layers are continuous, is the 

 only place where migration still goes on. 



It is important to note that the cubical, sharply defined, somatic 

 follicle cells of the later stage are separated from the cubical cells of the 

 follicle before fertilization, by an intervening period, when all follicle 

 cells show traces of active change and a share in the process of prolifera- 

 tion and migration. 



The study of the later stages shows that, during this period of active 

 change, the whole, follicle, winch was at first outside the egg, has become 

 internal to the embryo, and is now, morphologically, in its body cavity. 



I have given, in the section on the morphological significance of the 



