30 



Altogether there may he as many as seven distinct layers 

 around the mature egg, but they are all produced hy the 

 differentiation or activity of the follicle cells. 



Polar bodies are formed from the maturing ovum in the 

 usual manner, and effect the usual reduction in the number 

 of chromosomes in the nucleus. In the common Styelopsis 

 grossularia there are two chromosomes left in the ovum, 

 while in some species of Ascidia there are eight. 



The spermatozoa of Ascidia are of typical form (see PI. 

 v., fig. 1, s.). 



Embryology and Life-History. 



The egg (PI. V., fig. 1) after being fertilised (probably in 

 most cases by a spermatozoon carried by the current of 

 water from another Ascidian somewhere in the neighbour- 

 hood), proceeds to segment or divide into a number of 

 small pieces or young cells, thus becoming an embryo 

 Ascidian (PL V., fig. 2, &c.). The cells of the embryonic 

 body gradually come to be arranged (in a inanner the 

 details of which will be described below) so as to form— (1) 

 a skin or layer of cells— the ectoderm of the adult — covering 

 the outside ; (2) a tubular nervous system running along 

 the middle of the dorsal surface, miderneath the ectoderm; 

 (3) a short wide tube, placed ventrally, which is the 

 beginning of the branchial sac and the remainder of the 

 alimentary canal ; and (4) a cellular rod — the notochord — 

 which lies in the posterior part of the body, between the 

 dorsal nerve tube and the ventral alimentary canal, and is 

 a rudimentary or very simple back-bone, similar to that 

 found in the embryos of vertebrate animals (fig. 6, PI. V.). 

 In fact, the embryo Ascidian at this stage is comparable 

 with an embryo fish or frog, and is found to have the same 

 chief organs or parts similarly arranged ; and, moreover, 



