58 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



If the salpa embryo has ever had a food-yolk it undoubtedly agreed 

 in all respects with that of Pyrosoma and lay in the body cavity. 



It may, perhaps, seem to some that the incompleteness of the germ 

 layers of salpa may have been acquired as an adaptation to the presence 

 of the follicle or the placenta, and that it may therefore be quite inde- 

 pendent of the same phenomenon in Pyrosoma, but this view cannot be 

 seriously urged, for while we may believe that the ectoderm is potentially 

 present, in an undifferentiated state, in the surface of the yolk outside 

 the growing edge, as it certainly is in young amphibian embryos, we 

 cannot believe that it is potentially present in the follicle of salpa. 



I therefore think that we may safely assume, from the incomplete- 

 ness of the germ layers of Salpa, that its ancestors had a food-yolk like 

 that of Pyrosoma. 



The Primitive Salpa Embryo. 



Whatever view of the food-yolk we take, there can be no doubt 

 whatever that, far back in the past, the ancestor of both Salpa and 

 Pyrosoma had simple holoblastic eggs like those of Clavelina. This is 

 a necessary deduction from the principles of comparative embryology, 

 and it is also supported by the fact already pointed out that the salpa 

 egg shows the same type of segmentation as that of Clavelina. 



We may therefore safely assume, as the point of departure for our 

 comparative study of the embryo, a life-history like that of the primitive 

 chordata, where the holoblastic egg undergoes total regular segmenta- 

 tion and gives rise to a hollow blastula, from which a gastrula is formed 

 by invagination. 



From the dorsal part of the endodermal wall of the primitive diges- 

 tive cavity the notochord was formed, and the nerve tube arose from the 

 dorsal median ectoderm. At least one pair of ectodermal ingrowths 

 united with paired endodermal outgrowths from the pharynx, to form 

 the pharyngeal clifts through which the digestive cavity opened to the 

 exterior. The region of the tail elongated and the embryo became an 

 active locomotor chordate animal. So far the characters of the primitive 

 salpa embryo are common to the chordata. 



Furthermore we must assume that in the larval stage of all known 

 Tunicates, except Appeiidicularia, the ectodermal portions of the gill- 

 tubes moved towards the middle line, and united to form a common 

 dorsal cloaca, into which the digestive and reproductive organs came to 

 open. 



