IN GENERAL. O 



first accurately indicated by PURKINJE in the bird's egg, also con- 

 tains a nucleus, to which the name of germ-spot (macula germinativa) 

 has been given, and of which the discovery is due to R. WAGNER. 

 The investigations of the latest period have demonstrated the uni- 

 versality of these parts of the ovarian egg. When the egg passes 

 from the ovary into the oviduct, the germ-vesicle disappears, and is 

 replaced by a disciform germ-membrane from which the first 

 development of the embryo takes a beginning. In mammals, as 

 early as the latter end of the seventeenth century, the vesicles of 

 the ovaries had become known through the investigations of REG- 

 NERUS DE GrRAAF, which after their discoverer were named folliculi 

 graafiam, and were generally regarded as eggs. But only about 

 twenty years have passed since YON BAER found the true egg in 

 the ovary of mammals, which, however, he took for the germ-vesicle 

 then lately discovered by PURKINJE, and thought that in mammals 

 alone this vesicle passes from the ovary into the oviduct, whilst in 

 oviparous animals the entire egg, the yolk, is taken up by the 

 oviduct. Consequently, according to VON BAER, the folliculi graa- 

 fiani were to be regarded as ovarian eggs, which inclosed within 

 them the ovum fetale, the germ- vesicle 1 . It was afterwards 2 dis- 

 covered that this so-named ovum fetale also included within it a 

 germ-vesicle; in other words, that it was a perfect egg, which was 

 still further confirmed by the discovery of the germ-spot in this 

 vesicle. The folliculi graafiani are thus not eggs, but each of 

 them contains an egg which occupies only a small space of it, 

 but yet in its composition, in its inclosing a germ-vesicle with its 

 germ-spot, is similar in form to the egg in the ovary of birds and 

 other animals. 



On development the germ lies, as in invertebrate animals, in 

 form of a gelatinous disc on the yolk, immediately under the vitelline 

 membrane. The germ-disc separates into three layers; a serous 

 layer the most external, a mucous layer the most internal, and a 

 vascular layer between the two, but more intimately connected with 

 the mucous layer. In the middle part of the serous layer arise the 

 vertebral column, the spinal marrow and the cerebral mass. On 

 the outer surface of the serous layer arise two projecting lines, which 



1 C. E. A. DE BAER De (hi Mammaliuni et ffominis genesi. Lipsiie, 1827. 



2 COSTA, WHABTON JONES, VALENTIN, BERNHAKDT enz. 



