DEVELOPMENT DURING THE FIRST DAY OF INCUBATION 



209 



Fig. 123. A blastoderm of about 

 16 hours removed from the yolk 

 and stained. Various regions may 

 be distinguished by their abilities to 

 transmit light. The primitive streak 

 lies along the long axis of a clear 

 elliptical area. The clear area is the 

 area pellucida. Bounding the area 

 pellucida is the darker area opaca, 

 in which the blood islands are de- 

 veloping. At the anterior end of the 

 primitive streak Hensen's node ap- 

 pears as a somewhat denser spot. 

 The streak appears lighter along its 

 midline. A groove or depression — 

 the primitive groove — runs along 

 the midline. The future head struc- 

 tures of the embryo are located in 

 front of and around the anterior 

 end of the streak. (Courtesy Mr. 

 Lester A. Barter, Sloan-Kettering 

 Institute.) 



the blood islands from whence come erythrocytes and some blood vessels. 



Although no definite, permanent structure is yet visible, many of the 

 structures of the later embryo, such as heart and kidney, will self -differen- 

 tiate if the proper cells are transplanted at this time. In this respect the late 

 primitive-streak stage is similar to the late gastrula or early neurula of the 

 amphibian. 



From the time of the primitive streak to 24 hours the first definitive 

 structures become visible (Fig. 124, embryo about 3-3 mm. in length). The 

 primitive nervous system, which first arose as a flat plate, has now folded to 

 form the neural folds and neural groove. The neural folds have not come 

 together in the anterior and posterior levels but do approximate each other 

 in the middle regions. Under the head region the ectoderm and endoderm 

 are folded posteriorly to give a pocket (Fig. 101). This fold is termed the 

 head fold, and concomitantly with its formation the endoderm is folded back 

 upon itself to form a pocket which is the cavity of the foregut. The opening 

 into the foregut is the intestinal portal (Fig. 124). Figure 125 shows a cross 



