CHAP. X 



PODAXONIA 



373 



American species P. gouldii has been worked out by Gerould (1907), 

 who also confirmed his results by work done on the European species 

 P. vulgare. 



-The eggs of the American species, when they mature, are dehisced 

 into the coelom and pass into the nephridia. They are finally laid in the 

 sea and there fertilized. They are provided with a strong " chorion " 

 or " yolk-membrane," which persists until the close of embryonic and 

 the beginning of larval development. The spermatozoon penetrates 

 this membrane through a micropyle. After the eggs have been fixed 

 in picro-sulphuric acid, it is possible to dissolve the chorion by 

 exposing them to Laburraque's solution for two hours, and according 

 to Gerould no harm is done to the egg itself by this treatment. 



The cleavage reminds us in many ways of that of Dentalium. 

 The egg divides into the usual four macromeres, but D is, from the 



Fia. 301. — Early segmentation stages of the egg of Phascolosoma gouldii. (After Gerould.) 



A, 4-cell stage viewed from the side. B, 8-cell stage viewed from the side. 

 C, 8-cell stags viewed from above. A, chorion ; p.b, polar bodies. 



first, very much bigger than its sisters A, B, and C. It has, in fact, 

 five times the volume of any one of its three sisters. In the 8-cell 

 stage a first quartette of micromeres is formed, and these are 

 relatively large cells, as big as the smaller macromeres. 



In the 16-cell stage la, lb, Ic, and Id divide as usual into la^, 

 W-, lc\ and Id^, and la^ Ib^, Ic^, and Id^ respectively. These two 

 sets of cells are about equal in size, and they are larger than the 

 residual macromeres 2A, 2B, and 20. Of the second quartette of 

 micromeres which, with these macromeres, form the lower half of the 

 egg, 2a, 2b, and 2c are small, but 2d and its sister cell, the residual 

 macromere 2D, are both enormous and of about equal size. In 

 attaining the 32-cell stage, the upper eight cells of the egg divide 

 equally, so that the quartettes of cells lqi\ Iq^^ lq^\ and Iq^^ 

 are all of about equal size. So far as the second quartette of 

 micromeres are concerned, each divides into a small upper and lower 

 larger cell. The residual macromeres 2 A, etc., give off the third 



