CHAPTER I. 



The General Development of the Embryo. 



The Fertilised Ovum. — The youngest ovum found in the ovi- 

 duct is shown on PL I, fig. 1. It is of an elongated form — length^ 

 •4 mm. — and is surrounded by a transparent, structureless 

 membrane, within which is a second, more delicate membrane. 

 The outer membrane persists until birth ; it has a dense structure 

 and allows fluid to pass through it by diffusion. Water 

 diffuses through it more rapidly than alcohol, and alcohol 

 more rapidly than turpentine ; so that when an embryo is 

 removed suddenly from weak alcohol into strong, or from 

 absolute alcohol into turpentine, the membrane shrinks and 

 closely invests the embryo ; in fact, in the latter case all the 

 alcohol diffuses out before any turpentine enters, so that the 

 membrane completely collapses and squashes the embryo flat. 

 When, on the other hand, an embryo is removed from strong 

 alcohol into weak, or into water, the water passes in more 

 rapidly than the alcohol passes out, so that the membrane is 

 distended, and the space between it and the embryo much 

 enlarged. In the normal embryos there always is a space 

 between the membrane and the ovum, which contains fluid in 

 which the embryo floats. The membrane, therefore, has much 

 the same function as the amnion of the Vertebrata. 



The protoplasm of the ovum 2 is differentiated into two parts — 

 the main mass being of a pale colour with relativelv few dark 

 granules, while at one point it is especially dark in colour. This 

 small dark patch (fig. 1) is placed at the surface on one of the 



1 This refers to P. Balfouri; in P. capensis the average length is 

 •56 mm. 



* The following description of the segmenting ova refers, unless otherwise 

 stated, to fresh living ova seen in transmitted light. 



