10 EMBRYOLOGY. 



from each other in a high degree, so that they must really be con- 

 sidered as the most characteristic for the species of all the kinds 

 of animal cells. Their size, which is due to a greater or less ac- 

 cumulation of deutoplasm, varies so extensively that in some species 

 the egg-cells can be only barely recognised as minute dots, whereas 

 in others they attain the considerable dimensions of a Hen's egg, or 

 even of an Ostrich's egg. The form is usually globular, more rarely 

 oval or cylindrical. Other variations arise from the method in 

 which protoplasm and deutoplasm are constituted and distributed 

 within the limits of the egg ; there are in addition the differences of 

 the finer structure of the germinative vesicle and the great variability 

 of the egg-membranes. 



Some of these conditions are of great significance from their in- 

 fluence on the manner of subsequent development. They have been 

 employed as a basis for a classification of the various kinds of eggs. 



It is most expedient to divide eggs into two chief groups, into 

 simple and into compound eggs, the first of which is divisible into 

 several sub-groups. 



A. Simple Eggs. 



Simple eggs are such as are developed in an ovary out of a single 

 germinal cell. The eggs of all the Vertebrates and most of the 

 Invertebrates belong to this group. 



In this chief group there occur, according to the manner in which 

 protoplasm and deutoplasm are distributed within the egg, three 

 modifications, which are of very great importance in the determination 

 of the first processes of development. 



In the simplest case the deutoplasm, which ordinarily is present 

 only to a limited amount in the correspondingly small egg, is more 

 or less uniformly distributed in the protoplasm (fig. 1). In other 

 cases there has arisen out of this original condition, in conjunction 

 with an increase in the bulk of the yolk-material, an inequality in 

 the distribution of the two egg-substances previously distinguished. 

 The egg-plasma has accumulated in greater abundance at certain 

 regions of the egg -territory, and the deutoplasma at otJier regions. 

 Consequently, a contrast has arisen between portions of the egg-cell 

 which are richer, and those which are poorer, in protoplasm. A 

 further accentuation of this contrast exercises an extraordinarily 

 broad and profound influence on the first processes of development, 

 which take place in the egg after fertilisation. That is to say, 

 the changes, which further on are embraced under the process of 



