PROCESS OF DEVELOPMENT IN MAMMALIAN OVA. 535 



v. Baer, " embryonic stria," " embryonal spot " of Coste,) BischofF 

 was again the first to observe the occurrence of a division into 

 two laminae, and his description of the spot in question cor- 

 responds exactly to the relations that we have now, with 

 superior aids to research, ascertained to exist in the ova of 

 Birds, Batrachia and Fish. The cells of the animal layer, in 

 accordance with his description, form a dense membrane, whilst 

 the cells of the vegetative lamina are still distinct, and are very 

 delicate and pale. What signification we are to give to these 

 two laminae, in the present state of our knowledge, I am unable 

 to determine. A single tolerably clear section from the ovum 

 of a Dog of corresponding age led me to think that the two 

 laminae, of which the germinal vesicle external to the dorsal 

 rudiment (germ elevation, Keimhugel) is composed, are to be 

 regarded as the analogues of Remak's sensorial and glandular 

 laminae. It is reserved for the future to give a more definite 

 account, especially in regard to the relations existing at the 

 germ elevation. 



I am unable to make any further statement respecting a third 

 vascular lamina occurring between these two, which has been 

 much discussed, and which was admitted by Bischoff. It is, in 

 the first place, doubtful whether this middle lamina is really 

 a vascular lamina, or merely corresponds to Remak's middle 

 germinal lamina. The subject of the development of the middle 

 germinal lamina in Mammals requires a fresh investigation. 

 After the knowledge that has been gained by researches con- 

 ducted on other classes, I cannot venture to reproduce here any 

 of the opinions generally advanced. So far as this lamina 

 really contains the rudiments of vessels, I shall hereafter have 

 to refer to it again. 



(e.) MORPHOLOGICAL VALUE OF THE GERMINAL LAMINA. 

 It has already been stated that the external germinal lamina 

 (epiblast) contains the rudiment of the central nervous system, 

 of the nervous constituents of the organs of sense, and of the 

 superficial cellular investment of the animal. It has also been 

 explained why I have designated it the conjoined corneal and 

 nervous lamina. In Batrachia, in which the corneal lamina is 

 separated from the nervous lamina, the relations are extremely 



