EMBRYO WITH SEVEN SEGMENTS. 



303 



indicate clearly the formation of the endothelial heart (Fig. 158, Endo). At 

 first the cells are irregularly disposed and have several irregular cavities between 

 them which soon fuse so as to form two main cavities running longitudinally. As 

 the two cavities enlarge they meet in the median line and remain separated at 

 first by a wall of two layers of endothelium. This wall soon breaks through, and 

 there results a single median tube of endothelium which presently appears to be 

 connected with the mesothelium, m. ht, by long cell processes across the wide, 

 intervening space. The heart is now a double tube connected by the meso- 

 thelium with the tissues above. Still later we shall find the endothelial heart 

 enlarged and the muscular heart to have grown across on the dorsal side so as to 

 form a closed tube which separates finally on the dorsal side from the mesothe- 

 lium with which it is originally connected, and after this separation we have a 

 double free heart tube underlying the fore-gut. 



Fig. 174. — Chicken Embryo with Seven Segments. Transverse Section across the Primitive 



Groove. 



£c, Ectoderm, mes, Mesoderm. £ni, Entoderm. I'r.g, Primitive groove. The large black dots represent 



^olk-grains. X 230 diams. 



Sections through the Medullary Groove. — Figure 173 represents three sections 

 at different levels. In the first. A, the groove is quite deep and the young 

 primitive segment is shown. At the edge of the groove its thick walls pass over 

 continuously, but quite abruptly, into the general ectoderm, Ec, covering the 

 embryo. Close under the median line of the medullary groove appears an oval 

 section of the notochord, nch. The entoderm. En, is quite thin and somewhat 

 irregular, as is shown in all of the sections. In B the medullary groove is wide 

 open and quite shallow, the notochord is much larger and extends from the floor 

 of the medullary groove to the entoderm and occupies in part a deep notch in the 

 medullary wall. The notochord prevents the extension of the mesoderm across 

 the median line. In C the medullary groove is fading out and merging into the 

 beginning of the primitive streak, which forms a large mass of cells in the median 

 line in which the boundaries between the germ-layers cannot be determined. 

 Laterally this mass of tissue passes over into perfectly distinct germ-layers, of 



