EMBRYO WITH SEVEN SEGMENTS. 295 



tral cavity. The blood-vessels are formed solely by the endothelium (angio- 

 blast). There is nowhere any condensation of the mesenchyma about the blood- 

 vessels as yet. There are no capillaries whatever in the embryo. One of the 

 most important vascular modifications has, however, been initiated in the anlage 

 of the liver, where we find the vascular endothelium coming into close contact 

 with the entodermal cells of the liver, preparatory to the later complete differen- 

 tiation of the hepatic sinusoids. The blood-corpuscles are round in form with 

 fairly distinct outlines. Their protoplasmic bodies are much larger than those of 

 any other cells of the embryo at this stage, but their nuclei resemble in size and 

 structure those of other tissues. 



Embryo Chick with Seven Segments. (About twenty-seven hours' incubation.) 

 The following description is almost equally applicable to embryos with five 

 or nine segments. 



Examination in the Fresh State. The embryo when first removed from the 

 yolk should be placed in a staining-dish with a small quantity of normal salt 

 solution and examined with a low power of the microscope as a transparent 

 object. The specimen as a whole has a grayish or brownish-gray tint. Most 

 of the germinal area is dark, the transmission of light being stopped by the 

 numerous yolk-grains contained in the entodermal cells (compare page 87). In 

 the center of the germinal area the translucent area pellucida is very conspicu- 

 ous, and has an edge which is quite sharply defined, more so than after the speci- 

 men has been preserved. It is shaped somewhat like an elongated pear. In the 

 axial- portion of the area the embryonic structures are partially differentiated 

 (Fig. 167). It should be noted that this figure is taken from a hardened, not a 

 fresh specimen. The head of the embryo is protuberant and is of a bluntly 

 rounded form. It projects freely above the surface of the germinal area. Under- 

 neath the projecting head is a very clear area in which there is no mesoderm what- 

 ever. This is the pro-amnion, pro. am. On either side of the head are two char- 

 acteristic spaces, a. c. i>, the amnio-cardiac vesicles. The surface of the germinal 

 area rises somewhat dome-like over each vesicle. This is due to the fact that in 

 this region the ccelom is already very large and the splanchnopleure or upper 

 leaf of the germinal area is arched on either side of the embryo. The relations 

 may be more clearly understood from cross-sections (Fig. 170). The posterior 

 limit of the head is marked by an arching line, the concavity of which faces the 

 caudal end of the embryo. This line, fov, marks the position of the fo-uea car- 

 diaca. On the sides of the fovea, running forward toward the median line of the 

 embryo, one can distinguish two darker bands which represent the beginning of 

 the formation of blood-vessels growing in from the extra-embryonic region to join 



