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. His- 

 tology (Greek, lords, "a textile fabric") is the science of tissues, and his- 

 togenesis 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 modifica- 

 tions of a small number of fundamental tissues, the development of which 

 may now be considered. 



It has already been stated that a new individual begins existence as a 

 single cell, the fertilized ovum. This cell then divides by mitosis into a 

 pair of cells, Fig. 25, A; and these again divide, making a group of four, 

 Fig. 25, B. By repeated mitosis a mass of cells is produced, which because 

 of its resemblance to a mulberry, is called a morula (Fig. 25, C). Develop- 

 ment to this point is known as the segmentation of the ovum. 



A section through the morula of the rabbit is shown in Fig. 25, 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 cleft enlarges until the entire structure becomes 

 a thin-walled vesicle, within and attached to one pole of which is the inner 

 cell mass (Fig. 25, F). Cells from this mass gradually spread beneath 

 the outer layer until they form a complete lining for the vesicle. The 

 inner layer is called entoderm, and the outer layer ectoderm. 



Before the entoderm has encircled the vesicle, a third layer has appeared 

 between the other two. This middle layer is the mesoderm (Fig. 25, 

 G). It arises from the place where the ectoderm and entoderm blend 

 with one another. The layers may be separated and floated apart 

 except at this spot where they are "tied together." This place is there- 

 fore called the primitive knot. The mesoderm also spreads laterally from 

 a longitudinal thickening of the ectoderm, which extends backward from 

 the primitive knot and marks out the future longitudinal axis of the 

 embryo. This thickening is the primitive streak. Arising from the primi- 

 tive knot and primitive streak, the mesoderm spreads out rapidly between 

 the ectoderm and entoderm, and very soon it splits into two layers (Fig. 

 25, H). One of them (the somatic layer) is closely applied to the ecto- 



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