56 JOHNS HOPKINS UNIVEESITY MORPHOLOGICAL MONOGRAPHS. 



believe that the whole organization of the body, in the most complete 

 form which it had then attained, was latent or potential in the fertilized 

 egg, and was formed from it by cell multiplication. 



Furthermore, we may be sure that, for a time, all the chordata origi- 

 nally followed the same line in their ontogenetic development from the 

 egg ; that all the Tunicates retained a common life-history for a longer 

 time ; that the embryos of Salpa, Pyrosoma and Doliolum were originally 

 alike for a little longer, and that the differences between the embryos of 

 these three forms are the results of more recent modification. 



These statements involve no opinion on the exact character of the 

 relationship between the ordinary Tunicates on the one hand, and Salpa, 

 Pyrosoma and Doliolum on the other, and no opinion on this point is 

 necessary for our purpose, which is to study the morphological signi- 

 ficance of the salpa embryo. 



Has the Egg of Salpa passed through a Stage with a Large Food- Yolk? 



Pyrosoma has a big food-yolk and a disk-shaped blastoderm ; and 

 the arrangement of the germ layers and the anatomical relations of the 

 embryo are profoundly modified by its presence. Was this yolk acquired 

 before or after Pyrosoma and Salpa diverged from each other? 



The salpa egg has nothing of the sort now, and it undergoes total 

 segmentation, but if the yolk was acquired before the divergence of the 

 two life-histories, we must, in our comparative study of the salpa embryo, 

 take into consideration the inherited results of its existence in the 

 ancestors of this embryo. 



The question cannot be definitely answered, but the incompleteness 

 of the ectoderm and the slow closure of the floor of the pharynx of salpa 

 are what we might expect if a food-yolk has once been present. In an 

 ordinary gastrula the continuity of the germ layers is complete, but it 

 will be seen by comparing the figures in Plates XVI, XVII, XVIII and 

 Figure 11 of Plate XLII, that the ectoderm of salpa has a growing edge, 

 and that it gradually spreads out on all sides over the embryo until its 

 growth is stopped, in the older stages, by the placenta. The starting- 

 point from which the ectoderm spreads is on the middle line of the 

 dorsal surface of the embryo around the region which I shall soon give 

 my reasons for regarding as the place of the blastopore. 



In meroblastic eggs, like those, for example, of Teleosts and Birds, it 

 is well known that even after the germ layers are established they have 



