10 GENERAL CONCEPTIONS. 



A Summary of Embryological Development. 



The following summary applies to what is known of vertebrates only. It I 

 would require some modifications to be applicable to the whole animal kingdom. 

 Each individual arises froni a single cell which is termed the impregnated or fer- 

 tilized ovum. From this all embryological study starts. The fertilized ovum 

 has its earlier history, since it is the product of the fusion of two sexual elements. 

 It is a living cell, and therefore contains protoplasm and nucleus. It is also 

 furnished with a certain amount of material known as yolk, which exists in 

 the form of separate granules imbedded in the protoplasm. This__y_olk js the 

 reserve food material, and by the. assimilation thereof the protoplasm of the 

 ovum can grow. 



The first step in the development r, tht repeated division of the original cell 

 so that there is produced an increasing number- of cells. The earlier stages of this 

 cell multiplication are designated as the segmentation of the ovum. This name is 

 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 con- 

 venience. There are two principal types of ovum known: in one type w r e 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 exclu- 

 sively 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 importance 

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

 occupation of the student will be to familiarize himself with the appearance 

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

 which are produced from them. They dominate every phase of development, 

 the form of every organ, the production of every tissue. Their importance is 

 so great that embryology might almost be defined as the science of germ-layers. 



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

 ectoderm, epiblast; for entoderm, hypoblast; for mesoderm, mesoblast. I have preferred to maintain the older 

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



