264 JOHNS HOPKINS UNIVEES1TY MORPHOLOGICAL MONOGRAPHS. 



salpa embryo, my reasons for believing that the follicular structures, 

 which are colored blue in Plate X, Fig. 8, form a model or cast of the 

 inside of a gastrula, although, as the result of unexampled secondary 

 changes, the formation of the germ layers of the gastrula has been so 

 much retarded that it has not yet begun. 



It is important to note also that while all the follicle cells at the 

 stages shown in Plate X, Figs. 5 and 8, exhibit every sign of an active 

 share in the remarkable process of proliferation and migration, the 

 actual movement, from without inwards, is confined to a restricted area, 

 and it is also noteworthy that this area is considerably larger at first 

 than at a later stage. At the stage of Plate X, Fig. 3, the area where 

 migration is subsequently to take place includes about half of the 

 spherical follicle or all of the hemisphere on the right of 8, 8, while at 

 the stage of Fig. 8 migration is restricted to one pole, the one on the left 

 in the figure. All these facts have been referred to in the discussion of 

 the salpa embryo, and I have tried to show that they all help us to under- 

 stand its true significance. 



The subsequent history of the embryo shows that the area which is 

 marked 10 in Plate X, Figs. 2, 5, 6, 8 and 9, is at the posterior end of its 

 body near the middle line of its dorsal surface ; that is, its relation to the 

 body is the same as that of the blastopore of the chordata with an invag- 

 inate gastrula stage. 



We have traced the history of the follicle to the stage when its cavity 

 is obliterated, as shown in Plate XI, Fig. 2. The visceral portion, with 

 its contained blastomeres, now fills it and lies in direct contact with the 

 inner surface of the somatic layer, and the latter has become a thin, 

 sharply marked epithelium, consisting of a single layer of cubical cells 

 with distinct boundaries. 



Part of another section from the embryo shown in Plate XI, Fig. 1, 

 is shown very highly magnified in Plate XLII, Fig. 1. The somatic layer 

 is on the left, where its nuclei form a single row. The outer ends of its 

 cells form a continuous surface, while their inner ends are slightly 

 rounded. The borders between the cells are distinct. The vigorous 

 proliferation which took place at earlier stages has nearly ceased in the 

 somatic layer, and when a cell is occasionally found in process of division, 

 as at the top of the figure, this takes place by karyokinesis. The cells of 

 the visceral layer, on the other hand, have no distinct borders, and they 

 now begin to multiply very rapidly by direct division of their nuclei, 

 which soon assume a most characteristic structure. They elongate and 



