HISTOLOGY 



crease in number as new ones become cut off from the unsegmented 

 mesoderm in the posterior part of the embryo. At first each somite may 



Med. groove 



Prim, knot 



Fig. 27. Fig. 28. 



PIGS. 27 AND 28. RECONSTRUCTIONS OF A CHICK EMBRYO. INCUBATED APPROXIMATELY 30 HOURS. X3O. 



FlG. 27 represents a dorsal view. The ectoderm has been removed except around the rhomboidal sinus 

 and under the head. On the left side, all the mesoderm except the blood vessels has also been re- 

 moved; a. portion including nine somites remains on the right side. The lowest layer beneath the 

 vessels, is the entoderm. Fig. 28 is a median sagittal section, except that the entire heart has been 

 included. Ant. neur., anterior neuropore; Med. groove, medullary groove; Med. tube, medullary 

 tube; Mes. som., mesodermic somite; Opt. ves., optic vesicle; Oral mem., oral membrane; Peric. cav., 

 pericardial cavity; Pr. knot, primitive knot; Pr. str., primitive streak; R. sinus, rhomboidal sinus; 

 Vit. v., vitelline vein; W. duct, Wolffian duct. 



contain a cavity, which is an extension of the ccelom, but the cavity is 

 soon obliterated by a plug of cells. In dorsal view some of the somites 

 are shown on the right side of Fig. 27; the rest have been cut away. 



