NATURE STUDY IN SCHOOLS. 



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variable ia form in different animals, always consist of two parts, an en- 

 larged head and a very slender tail like organism. The head alcne appears 

 to contain the essential element for fertilization ; the tail is simply a vi- 

 brating or propelling organ. By various methods the developed spermatozon is 

 brought in contact with the ripe ovum and penetrates it. Without entering 

 minutely into the complicated changes which occur after the spermatozoon 

 has entered the ovum, I will briefly give the most prominent points. First, 

 the spermatozoon approaches the nucleus of the ovum and the two seem to 

 fuse together. Keally, however, both elements still remain separate, but be- 

 come intimately connected. Now ensues an intricate, though orderly process, 

 the result of which is the division of the apparently fused male and fe- 



Fig. 80. 



Diagram showing process of segmentation of egg. A, ripe ovum into which a spermatozoo 1, o, 

 is penetrating. B, segmentation begins and continues progressively through the flgures to F. 



male nucleus, and this nucleus division is followed by a division of the entire 

 ovum. Thus we find two cells where there was one before ( see fig. 80, k). 

 each of which will have its nucleus. But what is important for us to 

 note is that each of these nuclei contains an equal portion of the male and 

 female elements. The two cells again divide, producing four ( see c ) which 

 in turn redivide and produce eight ; ( d ) these produce sixteen, ( see e ) and so 

 divisions continue until the ovum is divided into numerous cells. ( See f). 

 The next change to be noted is the passing of the spherical ovum with its 

 multiple cells, into another form, and as it is at this point that there is a 



