Il 
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. Asan example we will use the egg of the sea-urchin, upon 
which the majority of experiments on the chemical activation 
Fic. 1.—Unfertilized egg of Fic. 2The same egg about 
the sea-urchin, S. purpuratus, two ees later, after the ote 
s .d by spermatozoa. trance of the spermatozoon anc 
urrounded by spe 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 
l7/ 
