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JOHNS HOPKINS UNIVERSITY MOEPHOLOGICAL MONOGEAPHS. 



cut K, although the salpa embryo exhibits other peculiarities which have 

 not yet been discussed. In the diagram the arrangement of the follicle 

 cells inside the body cavity is so much like the arrangement of a meso- 

 derm that I have called its outer and inner layers somatic and visceral, 

 after the analogy of the mesoderm. At the stage shown in the diagram 

 the embryo has a body wall made up of an outer layer of ectoderm, 3, 

 and an inner layer of somatic follicle cells, 7 ; and a digestive cavity 

 whose wall is made up of a layer of endoderm, 9, and a visceral layer 

 of follicle cells, 8, while its lumen is filled with follicle cells. The body 

 cavity is bounded on all sides by follicular epithelium. 



CUT K. 



Cur L. 



Now if we picture this gastrula as gradually shaping itself into a 

 Tunicate embryo, it is plain that, as the various organs are blocked out 

 by foldings and ingrowths and outgrowths of the germ layers, the folli- 

 cular epithelium of the body cavity will take part in all these changes, 

 and will thus conform to the shape of all the organs which are formed 

 before it degenerates. Thus all the organs of the embryo will become 

 outlined in both blastomeres and follicle cells, so that the follicle will 

 become a sort of mold or cast of the embryo. These changes are so 

 hard to draw and so easy to imagine that diagrams of them seem 

 uncalled for. 



The most remarkable peculiarities of the Salpa embryo are now to 

 be described. They are due to another set of secondary changes which 

 are quite anomalous. 



After the follicle had become developed in the way which I have 

 sketched, so that it formed a cast of the embryo, segmentation and the 

 formation of the germ layers became retarded gradually, step by step, as 

 compared with the development of the follicular layers, until the salpa 

 embryo as we now know it was evolved. Cut L is a diagram of a salpa 

 gastrula, and the character of its secondary modifications will be under- 

 stood by comparing it with cut K. 



