CHAPTER VII. 

 THE GENESIS OF TISSUES AND ORGANS. 



IN the last chapter we considered the General Embryology 

 of the frog, the fowl, and the rabbit. We saw how the so-called 

 germinal layers were constituted, how the body-cavity was 

 formed, how the vertebral plate was segmented into mesoblastic 

 somites, how 'the notochord and neural axis were formed, and 

 how the latter expands anteriorly into three cerebral vesicles 

 foreshadowing the fore-, mid-, and hind-brain. This, however, 

 carries us but a little way towards understanding how the tissues 

 and organs of the body take their origin. Their genesis, so far 

 as the space of one short chapter will permit, will now be con- 

 sidered. 



The cells of which the early embryo is composed are minute 

 nucleated masses of protoplasm. They are at first for the most 

 part undifferentiated ; but those of the epiblast and hypoblast 

 very early show signs of differentiation in the direction of 

 flattened or of columnar epithelium cells. The cells of the 

 notochord differentiate in a direction of their own, becoming 

 net-like and vacuolated ; while those of the mesoblast become, 

 in the course of development, differentiated into blood, blood- 

 vessels, muscle, connective tissue, cartilage, and bone, or are 

 instrumental in the production of these tissues. For it must be 

 remembered that, as we have seen in the chapter on General 

 Histology, there is in most tissues, besides the actual cells, a 

 greater or less amount of intercellular or of cementing substance, 

 which may, as in epithelioid tissue (endothelium), be compara- 

 tively insignificant, or may, as in cartilage and bone, be im- 

 portant. 



117 



