882 REPRODUCTION, DEVELOPMENT, GROWTH AND DEATH [CH. LIX. 



After the male pronucleus has formed in the substance of the 

 mature ovum, it approaches the female pronucleus, and when the 

 two pronuclei fuse, fertilisation is completed. The nucleus which 

 results from the fusion the first segmentation nucleus contains 

 the typical number of chromosomes, half being derived from the female 

 and half from the male germinal element. When the fertilisation is 

 completed, the segmentation nucleus is accompanied by two attrac- 

 tion spheres with their centrosomes (see fig. 564) ; one of these spheres 

 is introduced with the male pronucleus, and the other probably 

 originates from it by division, since the centrosome of the ovum is 

 lost during maturation. 



Loeb has suggested that the action of the spermatozoon is 

 essentially chemical, because in certain invertebrate animals (for 

 instance, sea urchins) he has been able to produce artificial partheno- 

 genesis by purely chemical methods. In his latest work, he placed 

 the ova in dilute acetic or formic acid ; by this means, a membrane 

 is formed upon the surface of the egg-cell as it is in normal 

 fertilisation; if the ova are then transferred to concentrated sea 

 water for a short time and then placed in ordinary sea water, they 

 segment and produce normal larvae. He considers that the sperma- 

 tozoon brings with it enzymes or other chemical substances which 

 excite the ovum in the same way as the chemical reagents mentioned. 



The changes by which the fertilised ovum is transformed into 

 the young animal may take place either inside or outside the 

 body of the parent. If they take place inside the parent, as in 

 mammals, including the human subject, the ovum is small, and 

 the nutriment necessary for its growth and development is derived 

 from the surrounding tissues and fluids of the mother. If the 

 development takes place outside the parent's body, as in birds, the 

 egg is larger; it contains a large amount of nutritive material 

 called the yolk, and it may, in addition, be surrounded by sheaths of 

 nutritive substance. Thus, in the hen's egg, the yellow part alone is 

 comparable with the mammalian ovum, and the larger part of that 

 is merely nutritive substance. Upon the yolk is a whitish speck, the 

 cicatricula, which is a small mass of protoplasm, about J of an inch 

 in diameter. In the cicatricula lies the nucleus or germinal vesicle, 

 and it is this small mass of protoplasmic substance which divides 

 and grows to produce the chick ; the yolk and the surrounding white 

 being used as food. 



Ova such as the hen's, in which only a small part, the cicatricula, 

 divides and grows, are called meroblastic. Small ova, with little food 

 yolk, such as the human ovum, divide completely during develop- 

 ment, and are called holoblastic, but numerous gradations occur 

 between the two extreme types. 



The further development of the individual systems of organs by 



