552 John Beard 



well defined form. Probably tliis is due to the effects of reagents. The 

 mature eggs, when squeezed out of the body in mass, are of a delicate 

 pinkish colour. In diameter they are in the average (J,036mmwith 

 germinai vesicle of 0,012 mm and germinai spot of 0,004mm diameter. 



I 



Spermatozoa. 



Under this head I will only say that one very often notiees quan- | 

 tities of undeveloped mother-cells of Spermatozoa in the vasa deferentia 

 of the male of M. glabrum. 



Fertilisation. 



The usuai prelimiuary to fertilisation bere as in other cases is the 

 formation of the polar bodies. Usually there are two in number. About 

 a quarter, more usually half an hour, after the ^^^ leaves the body the 

 first polar body is pinched off (fìg. 2). The t^% becomes somewhat 

 flattened at one pole, and a small perfectly clear vesicle is pushed out. 

 Shortly after the formation of the first the second polar body is also 

 formed, and the t^% is ready for fertilisation (fig. 3) . The second polar 

 body is often larger than the first. Among my notes is one to the effect 

 that the polar bodies appear sometimes to be formed after segmentation 

 has actually commenced. If this be the case it would lend some sup- 

 port to the view , advanced I think by one of the Professors Hertwig, 

 that the formation of the polar bodies is to be regarded as abortive at- 

 tempts at segmentation. After fertilisation the ^^^ becomes surrounded 

 by a membrane. 



Segmeutatiou. 



Segmentation usually commences three hours after the eggs are 

 laid. It is complete but unequal , and indeed follows the usuai Chaeto- 

 pod type. It has already been fairly described some 18 years ago by 

 Metschnikoff 1, but as there are some additions to be made, and in 

 order to give a complete picture of the whole course of development, 

 any recapitulation may not be out of place. Segmentation usually be- 

 gins in the artificially fertilised o,^'^ two or three hours after it leaves 

 the body. The fertilised egg lengthens somewhat, the usuai nucleus 

 spindle is formed, and division takes place. Two segments are formed, 

 one of which, viz. that at the animai pole of the egg is very much 

 smaller than the other , being in fact not more than a quarter the size 

 of the other. The yolk and yolk-nucleus are confined to the larger seg- 



' 1. e. p. 237. 



