116 DR. THOMAS ALCOCK ON THE 



is converted into that of the frog, and the fifth to the 

 complete absorption of the tail. 



First Period. — This includes the segmentation of the 

 yelk and the development of the embryo. According to 

 the present observations, the process of segmentation 

 occupied eight days, and seemed especially to linger during 

 the sixth and seventh days, when no change could be per- 

 ceived in the ovum. In the drawings representing the 

 successive stages of the formation of the embryo the 

 omission of the first stage is unfortunate, as it makes the 

 change abrupt from the mere spherical egg to a form with 

 bilateral symmetry, in which every portion of the yelk- 

 mass has taken a definite position in relation to the median 

 groove. Allowing for this omission, the series of drawings 

 of the embryo within the egg-membrane gives a fair repre- 

 sentation of the progressive advance in form and propor- 

 tion until the tadpole is ready to escape. 



The rudimentary structures which appear during this 

 period may be stated to be : — the cerebro-spinal axis and its 

 supporting skeleton, with the continuation of these struc- 

 tures in the large powerful tail ; the enclosing body-wall ; 

 the visceral arches; the organs of sense; the heart, and 

 the blood-vessels of the developing parts, which include 

 the middle dorsal line and fore part of the body, namely 

 the head and throat; and the general cutaneous system. 

 But the cavity of the abdomen remains filled, with undif- 

 ferentiated yelk-mass; and no commencement of the organs 

 which it afterwards contains is at present made. The 

 branchial arches are considerably advanced before the close 

 of the period ; and in this condition as regards the struc- 

 tures which are permanent throughout the life of the 

 tadpole the animal is hatched. 



Two other organs, however, have been formed, the use 

 of which is temporary and confined to the second period ; 



