OF THE TADPOLE. 35 



It is not my intention to give any account of the development 

 of the egg from its earHest stages, but thinking that a comparison 

 of its structure in a median stage with that when newly laid might 

 be interesting, a frog was killed in September, 1886, and sections 

 made of the ova then contained in the ovary. The ovum at this 

 date consisted of a nucleus surrounded by a thick follicle enclosed 

 in a layer of dark pigment (Plate I., Fig. i). The follicle con- 

 sists of masses of oval granules. The nucleus is pear-shaped, and 

 consists of an undifferentiated mass of protoplasm, which, under 

 the treatment employed to obtain sections, frequently had a semi- 

 granular appearance. 



At this point it would seem well to mention that the whole of 

 the sections spoken of in these papers, and of which diagrams 

 will be given, were prepared by gradual hardening in successive 

 strengths of spirit of 10 degrees, commencing with 20 degrees 

 and ending with absolute alcohol, next immersion in oil of 

 lavender, and lastly digesting at a temperature of 40 degrees C. for 

 some hours in a bath of soft paraffin. 



The nucleus at the date mentioned was not truly central, but 

 very nearly so, and the whole ovum, from pressure of neighbour- 

 ing ova, was distinctly angular, and indeed roughly triang'ilar or 

 pentagonal, no two having exactly a similar shape. Plate I., Fig. 

 I, A, B, c, gives an idea of their appearance, showing — (i) The 

 pigment layer, e\ (2) The thick layer, composed of oval granules, 

 g ; (3) the pear-shaped nucleus, ;/, which is nearly circular, as 

 seen in transverse section. 



When the ovum is first deposited, it consists of a dark ball, 

 the embryo, about the i/i4th of an inch (2 millimeters) in dia- 

 meter, enclosed in a jelly-like envelope, whicli to the eye appears 

 perfectly homogeneous, but if placed in dilute acetic acid (2 per 

 cent.) is seen to divide into two concentric layers, the inner one 

 distinctly marked off from the outer by its pale yellowish tint and 

 an apparent investing membrane. 



This is sometimes called the albuminous, but, more frequently, 

 the gelatinous envelope. An analysis of it made on March 7th, 

 just a week after deposition, showed that it was neither albumen 

 nor gelatine, but a substance which in its chemical characters par- 

 lakes rather of the nature of chondrin, differing, however, from 



