CHAPTER XV 

 EMBRYOLOGY 



Fertilisation, Segmentation and Germ Layer formation. 



THE study of Embryology, or the development of animals, may 

 well commence with the act that marks its inception, namely, 

 fertilisation, by which is understood the fusion of the nucleus of a 

 male gamete or spermatozoon with that of a female gamete or ovum, 

 i.e. the constitution of a new individual. The ovum, as we have 

 seen, is a large immobile cell often of great size, owing to its contained 

 yolk. It always contains a nucleus, termed the female pro-nucleus. 

 The sperm is little more than a male pro-nucleus with an accompany- 

 ing centrosome and a tail, whereby it is able to move more freely. 

 By some means or other, in those animals where fertilisation is 

 internal by an act termed copulation, the spermatozoa are brought 

 into proximity with the ovum. As a rule, numerous sperms sur- 

 round one ovum, and by means of their perforating caps, aided by 

 the active movements of the tail, start to bore their way into the 

 egg, which sometimes puts out a small receptive process to meet 

 one of them. Typically only one sperm penetrates the egg, whose 

 membranes then appear to undergo a rapid physical or chemical 

 change that prohibits the entry of further sperms. If several 

 should enter the ovum, as sometimes happens in large eggs, only one 

 of them is functionally active, and the others simply degenerate. 

 When the head has successfully entered the ovum, the tail is shed, 

 leaving only the head and neck, which are, therefore, to be regarded 

 as the essential parts. Once inside, the head enlarges to form a 

 typical nucleus with a chromatin network, and while so doing it 

 rotates in such a manner that the centrosome formed from the 

 granules in the neck, is pointed in the direction of the female pro- 

 nucleus. An aster arises around the centrosome, and this, together 

 with the male pro-nucleus, moves towards the egg nucleus. While 

 this is proceeding the chromatin granules of both pro-nuclei organise 

 to form a typical spireme, and one of two courses may be followed. 

 The more common way is for the centrosome and aster to divide 

 and the two poles to move apart, forming a spindle between them. 



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