xvi TRANSLATOR'S INTRODUCTION 



Along with the increase in bulk and distribution 

 of the nuclear matter, there goes an increase in 

 bulk and segregation of the ordinary protoplasm. 

 The simplicity of the actual development of most 

 back-boned animals is disguised by provision for 

 the nutrition of the growing embryo. In a large 

 number of cases, as, for instance, in birds and 

 reptiles, the egg-cell, a microscopic structure at its 

 first formation, is bloated out into the large eggs 

 with which we are familiar, by the addition of 

 quantities of food-yolk. These eggs, although 

 morphologically single cells, do not divide as cells. 

 A small disc of protoplasm, surrounding the 

 nucleus, floats upon the surface of the yellow yolk, 

 and, when the nucleus divides, furrows appear in 

 this between the daughter-nuclei, but stretch very 

 little way into the inert food-yolk. The subse- 

 quent marshalling of the cells is disguised by their 

 association with a preponderating mass of inert 

 material. In a far-distant period in the history of 

 evolution, the eggs of mammals like man were 

 large, and contained, as in the lowest existing 

 mammals, a store of food-yolk. Now the food- 

 yolk is not formed, as the developing embryo 

 obtains its nourishment from the blood of the 

 mother. But the course of development is dis- 

 torted, partly as a legacy from the old large-yolked 

 condition, and still more to suit the new method of 

 nutrition. Some of the simpler animals even 

 among existing vertebrates still exhibit a marshal- 

 ling of cells common among invertebrates, and to 

 be traced under the complications of higher forms. 



