notes. 475 



ferent form. The other seven now appear to be pretty well 

 established. 



91 (i. 285). The flat germ-disc of Birds, which even now, in 

 the opinion of most embryologists, represents the first starting- 

 point in the formation of the embryo, and to which all other 

 germ-forms have been referred, is, on the contrary, a late and 

 mnch modified germ-form, which has arisen in consequence of 

 the extension of the gastrula over the greatly enlarging nutritive 

 yelk. 



92 (i. 289). Site of Fertilization. In Man, as in other 

 Mammals, fertilization of the eggs probably usually takes place 

 in the oviduct : here, the eggs which, at the rupture of the 

 Graafian follicles, have emerged from the female ovary and 

 passed into the outer opening of the oviduct, meet with the 

 active sperm-cells of the male seed, which, during copulation, 

 penetrated into the uterus, and from there passed into the inner 

 opening of the oviduct. Rarely, fertilization occurs even on the 

 ovary, or not till within the uterus. (Cf. Chapter XXY.) 



93 (i. 293). The origin of the mesoderm in Mammals, as in 

 other animals, is, at present, among the most obscure and con- 

 tested points of Ontogeny. Remak, Balfour, and others derive 

 it from the entoderm, Kolliker and others from the exoderm. 

 Waldeyer, His, and others assert that both primary germ-layers 

 take part in the formation of the mesoderm. The last assump- 

 tion is, I believe, correct. (Cf. notes 76, 77.} 



94 (i. 297). The Germ-shield (Notaspis). The ordinary 

 view, that the germ-shield (= Remak's " Doppelschild ") is the 

 earliest rudiment of the actual embryo, results in many erroneous 

 conclusions. It is, therefore, necessary to point out especially 

 that the germ-shield represents the first well-defined central 

 dorsal part of the embryo. 



95 (i. 317). Body Wall and Intestinal Wall. The morpho- 

 logical distinction between the body wall and the intestinal wall, 

 certainly primordial, is probably referable to the simple primary 

 germ-layers of the Gastrasa. If the skin-fibrous layer is derived 

 from the exoderm, and the intestinal-fibrous layer from the 



