CHAPTER III. 

 THE MORPHOLOGICAL SIGNIFICANCE OF THE SALPA EMBRYO. 



A basis for the comparison of the salpa embryo with even its closest 

 allies is hard to find, for although it is still a true embryo and not a bud, 

 its early stages have been profoundly modified by secondary changes. 



Salensky holds indeed, (5) 396, that a knowledge of the develop- 

 ment of other animals does not help to clear up the obscurity which 

 involves the salpa embryo ; that its peculiarities are so different from all 

 that we know of other animals that we must not hope to bring it into 

 the general scheme of animal embryology, and that, while it begins its 

 development by the sexual method, this soon gives place to budding 

 from the wall of the follicle. 



I have shown that this view is untenable, and that the embryology 

 of salpa is not totally and fundamentally irreconcilable with the princi- 

 ples of general embryology, although it is quite true that the nature and 

 origin of the secondary changes are most perplexing subjects. 



As our starting-point in making comparisons, we may safely assume 

 that the fundamental plan of development was originally that of the 

 Tunicates in general, but we have very few facts to show the way in 

 which the complications were introduced. Salensky believes that the 

 embryology of Salpa bicaudata is less modified than that of the other 

 species, but only as regards accessory structures. As regards the 

 peculiar history of the blastomeres and migratory follicle cells this 

 species is like the others, and there is, so far as we know, no species 

 which presents a transitional stage in this history. Nor can we get 

 much help from Pyrosoma or Doliolum, the two nearest allies of Salpa, 

 for while the life-history of Doliolum, with its invaginated gastrula and 

 its tailed larva, is possibly more primitive than that of Salpa or Pyro- 

 soma, it presents no trace of the distinctive peculiarities of either of 

 them, and it therefore affords no better basis for comparison than the 

 ordinary Tunicates. 



The embryos of Salpa and Pyrosoma represent two distinct lines of 

 secondary modification, and neither serves as an explanation of the 

 other. 



