58 THE EIDDLE OF THE UNIVEESE. 



that time the little vesicles which are found in great 

 numbers in the human ovary and in that of all other 

 mammals had been taken for the ova. Baer was the 

 first to prove, in 1827, that the real ova are enclosed 

 in these vesicles — the " Graafian follicles " — and much 

 smaller, being tiny spheres one-120th inch in diameter, 

 visible to the naked eye as minute specks under 

 favourable conditions. He discovered likewise 

 that from this tiny ovum of the mammal there 

 clevelopes first a characteristic germ-globule, a hollow 

 sphere with liquid contents, the wall of which 

 forms the slender germinal membrane, or blasto- 

 derm. 



Ten years after Baer had given a iirm foundation 

 to embryological science by his theory of germ-layers 

 a new task confronted it on the establishment of the 

 cellular theory in 1838. What is the relation of 

 the ovum and the layers which arise from it to the 

 tissues and cells which compose the fully- developed 

 organism ? The correct answer to this difficult 

 question was given about the middle of this century 

 by two distinguished pupils of Johannes Muller — 

 Kobert Kemak, of Berlin, and Albert Kolliker, of 

 Wiirzburg. They showed that the ovum is at first 

 one simple cell, and that the many germinal globules, 

 or granules, which arise from it by repeated segmenta- 

 tion are also simple cells. From this mulberry-like 

 group of cells are constructed first the germinal layers, 

 and subsequently by differentiation, or division of 

 labour, all the different organs. Kolliker has the 

 further merit of showing that the seminal fluid of 

 male animals is also a mass of microscopic cells. The 

 active pin-shaped " seed-animalcules," or spermatozoa, 

 in it are merely ciliated cells, as I first proved in the case 



