66 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
Altogether there may be as many as seven distinct layers 
around the mature egg, but they are all produced by 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. 
Wi slie ws): 
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 manner 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, underneath 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 hes 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, Pl. V.).: 
In fact, the embryo Ascidian at this stage 1s comparable 
with an embryo fish or frog, and is found to have the same 
chief organs or parts similarly arranged; and, moreover, 
_— ) oa 
