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. SO). 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 Band 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 bifureated 
(Figs. 36 A and B), and we have the appearance of a sickle some- 
what similar to Ixoller’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. 
