DISCOVERY OF THE HUMAN EGG. 55 



Among the many and great services which Baer ren- 

 dered in detail to Ontogeny, especiall}' to that of Vertebrates, 

 his discovery of the human egg must be especially men- 

 tioned here. Most, even of the earlier naturalists, had 

 assumed that man proceeds, like other animals, from an 

 egg. The Theory of Evolution (pre-formation) had, more- 

 over, assumed that all past, present, and future generations 

 of the human race existed encased in the ova of Eve, the 

 common mother. Yet the ova of Man and other Mammals 

 were not actually known tiU the year 1827. For the egg 

 is exceedingly small, a spherical vesicle or bladder of only 

 one-tenth of a line in diameter, which can be seen with the 

 naked eye only under very favourable circumstances. This 

 spherical vesicle, when in the ovary of the mother, is en- 

 closed in a number of peculiar spherical vesicles of much 

 larger size, called Graafian follicles, after their discoverer 

 Graaf, and these were formerly universally regarded as the 

 actual eggs. It was not until the year 1827 — ^not fifty years 

 ago — that Baer proved that these Graafian follicles are not 

 the actual eggs, which are much smaller, and only imbedded 

 in the Graafian follicles. (Cf end of Chapter XXV.) 



Baer was also the first to observe the so-called germinal 

 vesicle of Mammals, that is, the little spherical bladder 

 which is first developed from the impregnated egg, and the 

 thin wall of which consists of a single layer of uniform 

 polygonal cells. (See Chapter VIII.) Another discovery of 

 Baer's, of great importance in understanding the types of 

 the lineage of the Vertebrates, and the characteristic 

 organization of this group of animals in which Man is 

 included, was that of the Chorda Dorsalis. This is a lono- 

 thin, cylindrical cartilaginous cord, which in aU Vertebrates 



