FORMATION OF EMBRYO. 657 



pores a spermatozoon might pass. It is by no means established 

 that such pores exist. However, in some way the spermatozoon 

 passes through the membrane into the protoplasm ; here its tail 

 disappears and the head assumes a spherical form, and to it the 

 name of " male pronucleus " is given. The male and female pro- 

 nuclei then unite to produce the fecundation nucleus. After this 

 occurs the ovum consists of a mass of protoplasm with a nucleus, 

 and is spoken of as the " segmentation sphere," because it under- 

 goes segmentation. 



Segmentation. This consists in the production of two seg- 

 ments by the same process of indirect division which takes place 

 in the germinal vesicle ; these again divide, forming four, and, the 

 same process continuing, the entire ovum is broken up into a mass 

 of spherical cells which, from the resemblance to a mulberry, is 

 named morula. These cells separate into two layers, with fluid 

 between them, except at one place where the layers are in con- 

 tact. The blastodermic vesicle is now formed. It is probable 

 that development has reached this stage at about the tenth day, 

 by which time the ovum has entered the uterus. The albuminous 

 secretion of the Fallopian tube serves as pabulum or food to the 

 cells in this process. 



Formation of Embryo. The next change which takes place 

 is the formation of three layers from the two just described. They 

 are termed the epiblazt, the mesoblast, and the hypoblast; together 

 they form the blastoderm. The epiblast is most external, in con- 

 tact with the vitelline membrane, which takes no part in the 

 changes thus far described. 



It would, perhaps, be too much to say that the embryo is now 

 formed, yet the subsequent changes are but the modification and 

 differentiation of the cells which compose these three layers. The 

 epiblast forms the brain and spinal cord, portions of the organs of 

 special sense, and the epidermis, and also takes part in the for- 

 mation of the chorion and amnion. The mesoblast forms the 

 vascular, osseous, and muscular systems, and the endothelium 

 which lines the serous cavities. The hypoblast forms the lungs, 

 the epithelium of the alimentary canal and of the glands which 

 are offshoots from this canal. The membrane which lines the 

 allantois and the yolk-sac is also formed from the hypoblast. 



The segmentation just described is such as takes place in the 

 human ovum and that of other mammalia. It is a process in 

 which the entire mass of protoplasm undergoes division : such 

 ova are said to be holoblastic. In the ova of birds and of 

 reptiles only a portion undergoes this segmentation, the rest serv- 

 ing as food. Such ova are meroblastic. As an illustration of the 

 latter may be mentioned the fowl's egg, in which the processes of 

 development have been most thoroughly studied. In this egg 

 only a minute portion, the cicatricula, becomes converted into 



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