II 



SOME REMARKS ON THE MORPHOLOGY OF 



DEVELOPMENT 



Since this book is intended not only for the zoologist, but 

 more especially for the physiologist, pathologist, and chemist, it 

 is necessary to give a sketch of the development of the animal 

 egg. As an example we will use the egg of the sea-urchin, upon 

 which the majority of experiments on the chemical activation 



6 a a 



:;-. . ''.!.. .'.;'.. *%' 



FIG. 1. Unfertilized egg of 

 the sea-urchin, S. purpuratus, 

 surrounded by spermatozoa. 



FIG. 2. The same egg about 

 two minutes later, after the en- 

 trance of the spermatozoon and 

 the formation of the fertilization 

 membrane. 



of development have been performed. The reason for this 

 is to be found in the fact that the eggs of the sea-urchin can 

 usually be obtained in large quantities and that they form 

 the most suitable material for our problem. 



Fig. 1 is a picture of the unfertilized egg surrounded by 

 spermatozoa. (The flagella of the spermatozoa have been 

 omitted in the drawing.) As soon as a spermatozoon has 

 entered, a very characteristic alteration takes place in the egg; 

 it becomes surrounded by the so-called fertilization membrane 

 (Fig. 2). The mechanism of this membrane formation can be 



17 



