26 SECOND LECTURE. 



lighter ones more slowly, the former arranging themselves 

 in rolls (e). 



We come, finally, to the blood formation of the embryo. 

 The fiat germinal layer, from which the human body arises, 

 consists of three membranous cell layers lying one over the 

 other, the horn layer (Hornblatt), the middle germinal layer, 

 and the intestinal gland layer (Darmdriisenblatt) (Remak). 

 The heart, vessels, and blood proceed from this middle layer, 

 from which, besides, many parts of the body originate. 



The first blood appears very early, and consists only of 

 colorless cells, formed of protoplasma and a vesicular nucleus. 

 The homogeneous yellow substance gradually replaces the 

 molecular protoplasma. We have now before us, nucleated 

 colored blood corpuscles (Fig. 18, a), of 0.0056 to 0.016 mm. 



At this period an increase also takes place by the way of 

 division (a to_/). Later, this procedure becomes extinct, and 

 the cells assume more and more the specific form, the nucleus 

 at the same time disappearing. 



Let us now proceed to the lymph and chyle. 



The fluid of the living blood, the plasma, constantly passes 

 through the thin capillary walls into the adjacent tissues. It 

 brings to the latter the nutrient materials, to the one these ; 

 to the second, again, others. The fluid becomes impregnated, 

 however, with the products of decomposition of the tissues. 

 The latter are again different. 



The tissue fluids, which are in consequence so variable in 

 their chemical constitution, finally collect in the fissures and 

 spaces of the body. Thin-walled vessels are gradually de- 

 veloped from these ; and then, uniting in larger trunks, they 

 finally enter the blood passages. These are the lymphatic 

 vessels; and the fluid contents, whose nature we have just de- 

 scribed, are called lymph. 



The walls of the intestines also have their lymph districts. 

 Towards the close of active digestion they contain tempora- 

 rily another cloudy or white fluid, which is very rich in albu- 

 men and fat. This is the lacteal juice or chyle. The canals 

 bear the name of the chyliferous system of vessels. 



