128 



that the generative organs are nui ripe simultaneously, 

 but that a verj^ short interval of time separates them. 



The development of P. opcrcuhiris has been worked 

 out by Fullarton (40j. He fertilised the eggs by 

 mincing the ovarian and seminal parts into two glass 

 vessels of sea water, which, after straining through 

 muslin, were mixed. 



The polyhedral egg cells soon as.^ume a spherical 

 shape, the time varying according to the state of 

 maturity, from a few minutes to half an hour. A vitelline 

 membrane is clearly visible, and a delicate hyaline 

 investment can be sometimes observed outside the 

 bounding wall of the ovum. A considerable quantity of 

 granular deutoplasm is also present, which partially 

 obscures the nucleus. The size of the ovum is O'OGS mm. 

 After fertilisation a polar globule is extruded, which in 

 many cases remains in contact with the oosphere for a 

 considerable time. The next visible change that takes 

 place is that the contents become much clearer at the 

 animal pole. This end is prolonged, so that the oosphere 

 becomes pear shaped, and eventually a constriction appears 

 and the etmbryo divides into a macromere and a micro- 

 mere, the latter occupying the animal pole. A second and 

 a third micromere are then budded oE from the macro- 

 mere, and so the four-celled stage is reached. The 

 micromeres now become active, and each sub-divides into 

 two, and they gradually spread over the surface of the 

 macromere until it is almost covered by them. The 

 macromere now begins to segment, two equal cells 

 resulting from the first division, which are the first tAvo 

 cells of the endoderm. These continue to divide, and are 

 completely enclosed by the micromeres, which form the 

 ectoderm and become ciliated ; the continuity is broken 

 at one point, however — the blastopore. The next 



