171 the Foetus of Vertehrated Animals. 305 



of the common frog, the yolk, occupying the central part, is 

 small and of a dark brown or blackish colour. At the time the 

 egg is laid, the yolk consists of a soft grey substance in its 

 centre, surrounded by a film of black granular matter, in which 

 the parts of the embryo first become developed (Fig. 2. c). 

 This layer of black matter, corresponding to the germinal mem- 

 brane, has not, in the ova of Batrachia, the appearance of a flat 

 disk, which it commonly presents in other animals, but is nearly 

 spherical in its form, and covers almost the Avhole of the proper 

 substance of the yolk, leaving uncovered only a small greyish 

 spot on the side of the egg opposite to that on which the foetus 

 begins to be developed (Fig. 2. o). 



We are indebted to the observations of Prevost and Dumas *, 

 Sir E. Home+, and Baer|, for our knowledge of the mode in 

 which the rudimentary organs of the tadpole are developed. 

 According to these authors, whose observations have been re- 

 peated and confirmed by Dr Sharpey and myself, the first 

 change which the germinal membrane undergoes is in its la 

 teral expansion ; it thus very soon fills up the vacant space 

 previously left on the surface of the yolk. The spinal 

 canal begins to be formed about a day after development has 

 commenced. A long dark eminence, bulged at one extremity, 

 makes its appearance on the upper surface of the germinal 

 membrane. This part is the primitive trace of the embryo, on 

 each side of which, a short time after, the germinal membrane 

 becomes thickened and raised, so as to form the primitive folds 

 or dorsal plates of the spinal canal (d d, Figs. 3, 4, 5). These folds 

 are broader in proportion to the size of the egg in the Batrachia 

 than in other animals ; they are thickest and most abrupt to- 

 wards the primitive trace. The yolk, at the same time, loses its 

 spherical form, and the germinal membrane is bulged out at 

 the places where the head and tail are afterwards formed. The 

 primitive folds are soon after united together behind, so as to 

 close the spinal canal ; and the rudiments of the vertebras now 

 appear on each side of it (Figs. 6, 7). 



We have already remarked that the germinal membrane 



• Annal. des Scien. Natur., torn. ii. 1884. 



+ Phil. Trans. Ifi25. Part i. 



X In Burdach's Physiologic ah Eliifahrungsgeswischenshaft, vol, ii. p. 222. 



