62 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



longer distinguishable from each other, and as he finds no cells in the 

 act of degenerating, he believes that they all help to build up the embryo ; 

 but there is every reason to believe that the follicle cells have the same 

 fate in Pyrosoma that they have in Distaplia and Salpa. 



While the Distaplia egg undergoes total segmentation, it is well filled 

 with yolk, and the segmentation cavity is thus obliterated; but this is a 

 secondary change, and we may picture the egg without it, as we may 

 also, in imagination, divest the Pyrosoma embryo and the Salpa embryo 

 of all secondary peculiarities except those which engage our attention ; 

 and the most satisfactory way to study the history of the follicle in these 

 three embryos is to strip them mentally of their irrelevant peculiarities 

 and picture them as reduced to their simplest expression, and to imagine 

 them as developing according to the primitive chordate type. I have 

 attempted to do this in the following diagrams. 



If we picture the Distaplia egg without its yolk, and with a spacious 

 segmentation cavity, it will be like cut I, in which 15 is the segmentation 

 cavity and 2 is the epithelial capsule of follicle cells. The space between 

 this and the embryo is filled with migratory cells, and these are repre- 

 sented as wandering into the substance of the blastomeres, and also 

 pushing their way between them into the segmentation cavity. 



In Distaplia the follicle cells are used up very early, but I have rep- 

 resented them in the diagram as persisting in great abundance at an 

 advanced stage of segmentation, as is the case in Pyrosoma and Salpa. 

 The blastula in the diagram is figured as about to undergo invagination 

 to form a gastrula, and the cells in the left hemisphere, 3, are ectodermal, 

 while those in the right, 7, are endodermal. For reasons drawn from 

 the embryology of Salpa, I have represented the migration into the sub- 

 stance of the blastoderm cells as most active in the endodermal hemi- 

 sphere, and the migration into the segmentation cavity as most active in 

 the equatorial zone where the two germ layers meet, and as more active 

 between the endoderm cells than between the ectoderm cells. 



If now we picture the invagination of a blastosphere like cut I, we 

 have a gastrula like cut J. As this stage is more advanced than the first, 

 the epithelial capsule of the follicle is represented as entirely broken up 

 into distinct cells, and while a few of these are still outside the embryo, 

 most of those which have not migrated into its substance are folded into 

 the digestive cavity as this is formed. A few of the migrating cells are in 

 the ectoderm, more in the endoderm, and others are pushing in between 

 the cells, especially around the edge of the blastopore, and wandering 



