1 8 HISTOLOGY. 



II. GENERAL HISTOLOGY. 



HISTOGENESIS. 

 SEGMENTATION AND THE FORMATION OF THE GERM LAYERS. 



The body is composed of groups of similarly differentiated cells, 

 similar therefore in form and function. Such groups are known as tissues. 

 Histology (Greek, IO-TOS, "a textile fabric") is the science of tissues, 

 and histogenesis deals with their origin. There are as many tissues in the 

 body as there are "sorts of substance"; thus the liver consists essentially 

 of hepatic tissue, and the bones of osseous tissue. All of these, however, are 

 modifications of a small number of fundamental tissues, the histogenesis 

 of which may now be considered. 



It has already been noted that a new human individual begins existence 

 as a single cell, the fertilized ovum, formed by the fusion of two mature 

 sexual cells, the spermatozoon and ovum respectively. The fertilized 

 ovum then divides by mitosis into a pair of cells, Fig. 19, A; these again 

 divide making a group of four, Fig. 19, B; by repeated mitosis a mulberry- 

 like mass of cells results, called the morula, Fig. 19, C. Development to 

 this point is known as the segmentation of the ovum. 



A section through the morula is shown in D. An outer layer of cells 

 surrounds the inner cell mass. Soon a cup-shaped cleft, crescentic in 

 vertical section, forms between the outer and inner cells as shown in E, and 

 this enlarges until the entire structure becomes a single-layered, thin-walled 

 vesicle, within and attached to one pole of which is the inner cell mass. 

 This mass gradually spreads beneath the outer layer until it forms a 

 complete lining for the vesicle, which becomes consequently two layered, 

 Fig. 19, G. The inner layer is called entoderm, and the outer layer, 

 ectoderm* The entire embryonic structure at this stage is called a 

 blastodermic vesicle. 



On the upper surface of the vesicle the future axis of the embryo is 

 indicated by a thickened streak called the primitive streak. In front of 

 this there is a groove in the ectoderm, also in the axial line of the future 

 body. It is named the medullary groove, and just beneath it is a rod 



* The ectoderm is in part derived from the superficial cells of the inner cell mass, and 

 in part from the primary outer layer of the vesicle. The former portion is to cover the body 

 of the embryo, and the latter [named trophoblast] covers the fetal membranes. These 

 membranes are to be described in a later chapter. They are omitted in the diagrams of 

 Fig. 19. 



