FROM LAYING TO FORMATION OF FIRST SOMITE 73 



brates (Figs. 35, 36, 44, etc.). The primitive groove does not 

 involve the extreme anterior end of the primitive streak, which 

 forms a little knot in front of it, the primitive knot (" Hen- 

 sen's knot"). The posterior end of the primitive streak termi- 

 nates in an expansion which is not very obvious in surface view, 

 and hence is not usually described; it may be called the primitive 

 plate (Figs. 36, 44 A, 44 B, etc). In some cases the primitive 

 streak and groove are bifurcated at the posterior end (Fig. 44 B). 

 The primitive streak is the first clear indication of the axis of the 

 embryo. 



The neurenteric canal is a canal that connects the posterior end of 

 the central canal of the neural tube with the intestine. It arises from 

 the anterior end of the primitive mouth, and is typically developed in 

 Selachia, Amphibia, reptiles, and some birds (e.g., duck, goose, Sterna, 

 etc.). It begins in the primitive pit and extends forward into the head- 

 process (p. 80). Subsequently the primitive pit becomes surrounded by 

 the medullary folds, and thus opens into the neural canal. An opening is 

 later formed through the entoderm so that the definitive canal connects 

 neural tube and hind-gut. In the chick the neurenteric canal is never 

 typically developed. Usually it is represented only by the primitive pit. 

 In exceptional cases I have found traces of it in the head-process. 



The so-called head-process appears in front of the primitive 

 knot (Figs. 36 B and 44 B). In surface view it appears not unlike 

 the primitive streak itself, but is fainter and less clearly defined. 

 It is continuous with the primitive streak at the primitive knot, 

 but its axis is usually a little out of line with the axis of the primi- 

 tive streak. 



Figs. 35 and 36 exhibit four stages of the development of 

 the primitive streak of the sparrow (after Schauinsland). The 

 darker area in the anterior part of the area pellucida is caused 

 by a thicker region of the entoderm which in the course of time 

 becomes of uniform thickness with the remainder. It will be ob- 

 served that the primitive streak arises entirely within the area 

 pellucida (Fig. 35 B). In later stages its posterior end is bifurcated 

 (Figs. 36 A and B), and we have the appearance of a sickle some- 

 what similar to Koller's description for the chick. The primitive 

 groove begins near the anterior end of the primitive streak in an 

 especially deep pit just behind the primitive knot, and extends 

 back the entire length of the primitive streak into the horns of 

 the sickle. The head-process is barely indicated in Fig. 36 B. 



