22 



THE HENS EGG. 



[CHAP. 



It is not certain whether impregnation takes place previous to the deposition 

 of the albumen, or whetber the spermatozoa bore their way through tbe albumen. 

 The former would appear to be the more probable view, though the fact that 

 Oellacher has found spermatozoa in the albumen, speaks in favour of their being 

 involved in the depositing albumen, and so being brought in contact with tue 

 blastoderm. 



According to Coste, Ilistoire du developpement des corps organizes, the access 

 of the cock to the hen once in seven days is sufficient. 



We have no positive evidence that the spermatozoa make 

 their way through the vitelline membrane and so gain access 

 to the germinal, disc; but, as will be seen in a later part of 

 this work, analogy renders such an event probable. 



14. At about the time when the shell is being formed 

 round the esfg, the germinal disc undergoes a remarkable 

 change, known as segmentation. We shall have occasion to 

 treat more fully of the nature of segmentation when we 

 come to consider the amphibian ovum in which the various 

 steps of the process may be more easily and satisfactorily 

 traced. Meanwhile, inasmuch as the segmentation of the 



ABC 



Surface Views of the early Stages of the Segmentation in a 



Fowl's Egg. (After Coste.) 



A represents the earliest stage. The first furrow (b) has begun to make its 

 appearance in the centre of the germinal disc, whose periphery is marked by 

 the line a. In B, the first furrow is completed right across the disc, and a 

 second similar furrow at nearly right angles to the first has appeared. The disc 

 thus becomes divided somewhat irregularly into quadrants by four (half) furrows. 

 In a later stage (C) the meridian furrows b have increased in number, from 

 four, as in B, to nine, and cross furrows have also made their appearance. The 

 disc is thus cut up into small central (c) and larger peripheral (d) segments. 

 Several new cross furrows are seen just beginning, as ex. (jr. close to the end of 

 the line of reference d. 



