66 



ORIGIN OF CHORDATES 



in. 4-5 



neural gland or of mammalian gonadotropin, but these act through the 

 ganglion, since they produce no effect if the nerves leading from this 

 (and the dorsal strand) are cut. 



Further similarities with the pituitary have been claimed, such as 

 the presence of vasopressor and oxytocic substances in the subneural 

 gland. However, oxytocin is present elsewhere in the tunicate and in 



Fig. 36. Longitudinal section of the ganglion (g.) and subneural gland 

 (s.n.g.) of an ascidian. 



cil. ciliated funnel; d.s. dorsal strand; n.a. and n.p. anterior and posterior nerves; 

 ph. wall of pharynx. (After L. Bertin from Grasse.) 



any case differs from that of vertebrates. It cannot be claimed that the 

 relationship with the pituitary is clear, but it seems likely that there 

 is some. As in the thyroid, a pharyngeal mucus-secreting organ 

 stimulated by the environment has evolved into a glycoprotein- 

 secreting endocrine organ, controlled by substances reaching it in the 

 blood. (Barrington, 1959, in Gorbman, Symposium on Comparative 

 Endocrinology.) 



5. Development of ascidians 



Tunicates are hermaphrodite, the ovary and testis being sacs lying 

 close to the intestine and opening by ducts near the atriopore. Fer- 

 tilization is external in the solitary forms but internal in those that 

 form colonies, the development in the latter taking place within the 

 parent. The details of cleavage and gastrulation show a remarkable 

 general similarity to those of amphioxus. Indeed, the whole develop- 

 ment is so strikingly like that of chordates that it establishes the 

 affinities of the tunicates far more clearly than the vague indications 



