454 THE EVOLUTION OF MAN. 



extension, the sexual glands originate from a portion of 

 the ccBlom-epithelium (Fig. 160, G). In other respects, the 

 farther modification of the larva into the adult form of the 

 Amphioxus is so simple that we need not now follow it.^^^ 



We will now turn to the history of the development of 

 the Ascidian, an animal apparently so much lower and so 

 far simpler in its organization, which spends the greater 

 part of its life as an unshapely mass, adhering to the bottom 

 of the sea. It was most fortunate that Kowalevsky in his 

 researches first fell in with those larger Ascidian forms 

 which most clearly testify to the kinship between Verte- 

 brates and Invertebrates, and of which the larvse, in the 

 first stages of development, are exactly similar to those of 

 the Amphioxus. This agreement in all the essential charac- 

 ters is so great that it is really only necessary to repeat 

 word for word what has already been said about the 

 Ontogeny of the Amphioxus. 



The egg of the larger Ascidia (Phallusia, Cynthia, etc.) 

 is a simple globular cell x^ to § mm. in diameter. In the 

 cloudy, finely granular yelk a bright, globular germ-vesicle 

 (nucleus) about ^ mm. in diameter is seen, enclosing a 

 germ-spot (nucleolus). (Fig. 1, Plate X.) Within the enve- 

 lojDc, which surrounds the egg, the parent-cell of the 

 Ascidian, after fertilization, passes through exactly the 

 same changes as the cytula of the Amphioxus. The special 

 incidents in the fertilization and egg-cleavage of the largest 

 and most interesting of our Ascidians (Phallusia mam- 

 milata) have lately been very accurately studied and 

 described by Edward Strasburger. The remarkable details 

 of these processes, which do not, however, touch our present 

 purpose, are given in the excellent work by that writer 



