SEA-FISHERIES LABORATORY. 455 
that the generative organs are not ripe simultaneously, 
but that a very short interval of time separates them. 
The development of P. opercularis has been worked 
out by Fullarton (40). Ile fertilised the eges 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 assume 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 0'068 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 embryo divides into a macromere and a micro- 
mere, the latter occupying the animal pole. A second and 
a third micromere are then budded off 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 two 
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 brcken 
at one point, however—the blastopore. The next 
