TISSUES OF THE BODY. 131 



Like the red cells, they also (but by virtue of their vital contractility) 

 pass through the uninjured walls of the vessels (fig. 124) in the healthy 

 as well as the diseased body ; in some cases re-entering the lymphatic cir- 

 culation, in others penetrating into various tissues ( 49). Now it is 

 probable that partly, at least, from this source the wandering lymphoid 

 cells of connective tissue have their origin. These we will make the sub- 

 ject of later consideration. Under conditions of inflammatory irritation 

 such an exit from the blood-vessels in the vicinity of the affected part 

 takes place on a large scale (A. Waller, Cohnheim), and the pus-corpus- 

 cles appearing at this spot are, in part, nothing but the lymphoid cells of 

 the blood which have emigrated. 



Finally, touching the origin of the blood in the embryo, we may pre- 

 mise by stating that we are but partially acquainted with this chapter of 

 histogenesis. 



But in order properly to comprehend the process, we must first render 

 ourselves familiar with the broader and more important outlines of 

 embryology generally. 



By the processes of segmentation in the impregnated ovum a cellular 

 material is formed which represents the germ, i.e., that spot at which the 

 body of the coming being is to be built up. It is first disposed in the 

 form of a membrane which, according to Remaps admirable investigations 

 (recently questioned, however), may be distinguished as made up of three 

 layers of cells arranged one over the other, from each of which certain 

 distinct tissues and organs have their origin. Thus we have a key to a 

 scientific classification of the tissues of the body. 



For the present it need only be borne in mind that the upper stratum 

 bears the name of the " corneous" the lowermost that of the "intestinal 

 gland " layer (Darmdriisenblatt). The derivatives of each of these will be 

 met with presently. From the intermediate leaf known as the "middle ger- 

 minal" layer very many structures take their origin; thus, the whole of the 

 large connective-substance group, the voluntary and unstriped muscles, 

 the vascular and lymphatic systems with their accessory organs and con- 

 tents, including the tissue under present consideration, the " blood" 



The first formation of blood then takes place at a very early period 

 in foetal life. But the primary blood-cells are not related in any way to 

 the characteristic corpuscles of later times, they are nothing but what 

 are known as the ordinary formative or embryonic cells of which originally 

 the most widely different structures of the body may consist. . 



The first appearance of the primary blood-cells corresponds with that 

 of the heart and larger vessels immediately adjoining it. Both of the 

 latter are said not to be originally hollow, but solid aggregations of cylin- 

 drical form, consisting of cells. Now the destiny of these cells entering 

 into the formation of the cylinders is various ; the peripheral become ad- 

 herent to one another, or unite more closely still to represent the primitive 

 walls of the heart and vessels, while between the most internal, bordering 

 on the axis, fluid gradually accumulates in such quantity as eventually to 

 immerse the cells completely. 



From this moment on we may with propriety speak of blood in the 

 embryo, in that the fluid in the rudimentary heart and vessels represents a 

 scanty plasma, and the cells suspended in it the primitive blood-corpuscles. 



At first the latter appear, as has been already mentioned, in the form 

 of plain spherical cells, with finely granular protoplasm, vital contractility, 

 and frequently vesicular nuclei, within which nucleoli may be seen. They 



