26 GENERAL CONCEPTIONS. 



due to the fact that the process was first observed in the eggs of amphibia in the 

 early part of the last century, before the cell doctrine had been established. In 

 default of a better name, the separate cells into which the ovum divided were 

 called segments, for it was, of course, not known that they were cells. Although 

 this term is no longer appropriate, it is still universally used because of its conve- 

 nience. There are two principal types of ovum known: in one type we find 

 only a small amount of yolk material; in the other a very large amount. There 

 are ova known intermediate between these two types. When the ovum is of the 

 first type, the whole of it undergoes segmentation at once, and to such an ovum 

 the term holoblastic is applied. In the second type, on the contrary, we find that 

 the protoplasm tends to accumulate at one pole of the cell and the yolk granules 

 at the other. The protoplasmic portion exhibits a far more active cell division 

 than the yolk-bearing portion, so that the segmentation seems to take place 

 exclusively around one pole or part of the ovum, which is, therefore, designated 

 as meroblastic. After the segmentation of the ovum the multiplication of the 

 cells continues, and they gradually arrange themselves in such a manner as to 

 form three distinct sheets or laminae, which are named "germ-layers." These 

 layers are designated: the outermost as Ectoderm, the innermost as Entoderm, 

 and the middle as Mesoderm* From an embryological point of view the im- 

 portance of these three primitive germ-layers cannot be over-emphasized. The 

 principal occupation of the student will be to familiarize himself with the appear- 

 ance of these layers and the modifications which they undergo, and the adult 

 tissues which are produced from them. They dominate every phase of develop- 

 ment, the form of every organ, the production of every tissue. Their impor- 

 tance is so great that embryology might almost be defined as the science of 

 germ-layers. 



The primitive germ-layers consist of very simple cells, and are themselves 

 at first extremely simple in their organization. The majority of the cells which 

 they contain undergo a greater or less degree of modification as development 

 progresses. This modification is termed differentiation, and is more fully con- 

 sidered in our next section, on Cytomorphosis. It is probable, however, that a 

 certain number of the cells very early in the development are set apart, preserv- 

 ing the primitive character of their protoplasm and taking no share in the forma- 

 tion of the tissues of the body. These cells, comparatively unmodified, are 

 known as the qerm-cclls. Their significance is more fully explained in the section 

 on Heredity. As the remaining cells form part of the body of the individual, 



* Some English and occasionally Continental authors use other terms for the germ-layers: namely, fur 

 ectoderm, eniblast ; for entoderm, hypoblast ; for mesoderm, mesoblast. 1 have preferred to maintain the older 

 terms which have been in almost universal use for a century. 



