DURATION OF EMBRYO DEVELOPMENT. 31 



From now until early morning the lessening of the 

 gastrula-mouth proceeds but slowly. 



Several other successive stages which I noticed in 

 the development advanced with similar speed. 



On the morning of the first day after, that is, the 

 lapse of ten hours the gastrula-mouth was still wide 

 open. The closing of it is a very slow process as 

 compared with the other processes of development, 

 and occupies, as a general rule, the greatest part of 

 the forenoon. 



From, noontide of the first day till evening the most 

 important processes of development advance with 

 great speed, I refer to the remarkable folding of the 

 primary hypoblast which introduces the formation of 

 the rnesoblastic somites and of the notochord, as well 

 as the formation of the neural canal from the epi- 

 blasfc. So we have a succession of important processes, 

 through which the most important systems of organs 

 are developed from the two primary germ, layers. 



In the next twenty-four hours nothing further took 

 place beyond a slight increase of the niesoblastic 

 somites, elongation of the form, of the body, and histo- 

 logical differentiation. So far as new organs are 

 concerned, there is only to be noticed in this period the 



