THE HEMIMYAKIA (SALPIDAE) CLEAVAGE. 



419 



(SALKNSKY) that the embryonic development of Salpa bicaudata does not, in 

 essentials, diverge greatly from that of other forms. The greater the increase 

 in size of the embryo and of the placenta which has attached itself at the base 

 of the tube, the shorter does the tube become. The embryo finally passes into 

 the-atrial cavity of the parent through the opening of the tube (Fig. 209 B). 



The first change in the genital apparatus which precedes the 

 fertilisation of the egg, occurs in quite young Salps, in the act of 

 detaching themselves from the stolon of the " nurse " or just after 

 detachment. It consists of a continuous shortening of the oviduct, 

 especially affecting the part that has been described as the stalk of 

 the follicle, the cells of which shift towards each other in such a way 

 that they soon form several layers, while a lumen appears, so that 



e' 



l-'n;. 210. Two outogenetic stages of Salpa pinnata (after SALENSKY). In A, the 

 embryo consists of four blastomeres, two of which are cut through in the section. 

 In B, it consists of a larger number of blastomeres and of numerous smaller cells, kz. 

 f>. lilastomeres ; bl, blood-sinus ; e, epithelial prominence ; e', modified part of the 

 epithelial prominence (SALENSKY'S ectoderm-germ) ;fh, enveloping fold ;fw, follicle- 

 wall ; fz, immigrating follicle-cells, kalymmocytes ; kz, smaller cells of the embryo 

 (SALENSKY'S gonoblasts) ; o, shortened oviduct. 



the oviduct is now hollow throughout its whole length. During this 

 abbreviation of the oviduct (Fig. 210) the egg, together with the 

 follicle, shifts continually nearer the aperture of the duct, the process 

 M investing that seen in the descent of the mammalian testes. 



Through the now open oviduct the spermatozoa obtain access to 

 the follicle, and fertilisation takes place. According to TODABO (No. 

 112), the spermatozoon enters the egg and the male pronucleus 

 develops after the expulsion of the first polar body and before the 

 development of the second. 



Cleavage is total (Figs. 210 A,. 211 A and B). Eggs have been 

 observed divided into two and into four and also in the later stages 



