DEVELOPMENT OF VERTEBRATES. 115 



The mode of formation of the mesoblast is not certainly 

 known in Amphioxus ; we shall find, however, that for all other 

 vertebrates it arises from the cells which are homologous with 

 the involuted cells of this animal. 



Since food material is a term which will be very often em- 

 ployed, it will be well to explain exactly the sense in which 

 it will be used. It will be used only with reference to those 

 passive highly refractive particles which are found embedded in 

 most ova. 



In some eggs, of which the hen's egg may be taken as a 

 familiar example, the yolk-spherules or food material form the 

 larger portion of the ovum, and a distinction is frequently made 

 between the germinal disc and the yolk. 



This distinction is, however, apt to lead to a misconception 

 of the true nature of the egg. There are strong grounds for 

 believing that the so-called yolk, equally with the germinal disc, 

 is composed of an active protoplasmic basis endowed with the 

 power of growth, in which passive yolk-spherules are embedded ; 

 but that the part ordinarily called the yolk contains such a 

 preponderating amount of yolk-spherules that the active basis 

 escapes detection, and does not exhibit the same power of 

 growth as the germinal disc. 



With the exception of mammals, whose development requires 

 to be more completely investigated, Amphioxus is as far as we 

 know the only vertebrate whose ovum does not contain a large 

 amount of food material. 



In none of these (vertebrate) yolk-containing ova is the food 

 material distributed uniformly. It is always concentrated much 

 more at one pole than at the other, and the pole at which it is 

 most concentrated may be conveniently called the lower pole of 

 the egg. 



In eggs in which the distribution of food material is not 

 uniform segmentation does not take place with equal rapidity 

 through all parts of the egg, but its rapidity is, roughly speaking, 

 inversely proportional to the quantity of food material. 



When the quantity of food material in a part of the egg 

 becomes very great, segmentation does not occur at all ; and 

 even in those cases where the quantity of food yolk is not 

 too great to prevent segmentation the resulting segmentation 



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