24 BIOLOGY OF THE LABORATORY MOUSE 



be discussed later, but the reader will do well at this stage to look ahead to 

 Figs. 19 and 20 which show the way in which it deepens and narrows and 

 ftnally closes at the top to form the neural tube. The point to be empha- 

 sized here is that the appearance of the neural groove establishes a perfectly 

 clear caudal-cephalic axis throughout the length of the embryo. The neural 

 groove anteriorly and the primitive streak posteriorly lie in the precise 

 mid-plane and together separate the right and left halves of the embryo. 



The notochord. — At the same time that the neural groove is differentiat- 

 ing in the mid-sagittal area of the ectoderm, changes are also going on in the 

 mid-sagittal region of the head process which immediately underlies it 

 (Fig. 14). In this region the head process thickens, and the oval nuclei 

 become oriented in general perpendicular to the ectoderm. Elsewhere it 

 forms a thin membrane with the nuclei oriented parallel to the plane of the 

 membrane. The structure thus differentiated ventral to and in contact 

 with the ectoderm of the neural groove is the notochord. It is the axis about 

 which the vertebral column is later laid down. The remainder of the head 

 process, together with a part of the entoderm to which it is fused, becomes 

 the lining of the gut.* This part of the head process will hereafter be 

 referred to as gut entoderm. For a considerable period notochord and gut 

 entoderm remain joined. Eventually, however, the two halves of the gut 

 entoderm grow across the ventral surface of the notochord and unite in the 

 mid-ventral line, leaving the notochord as an axial, rod-like structure 

 between ectoderm and entoderm. 



Huber (23) describes the head process in the guinea pig as giving rise to 

 notochord only. Our material, however, confirms completely the conten- 

 tion of Jolly and Ferester-Tadie (26) that in the mouse at least some gut 

 entoderm is also derived from the head process. The critical stage is that 

 shown in Fig. 14B in which it can be seen that the head process extends 

 laterally considerably beyond the limits of the differentiating notochord. 



A much mooted question is whether the notochord should be classed as 

 ectoderm, entoderm or mesoderm (31). Since it is formed from the head 

 process and since the ver>' complete fusion of the margins of the head process 

 with the entoderm indicate a close affinity between the two tissues, classi- 

 fication as entoderm would seem logical. If, however, head process is 

 classed as entoderm, it must be remembered that its origin in time is quite 

 different from that of all the other entoderm, and two separate stages of 



* It seems likely that most or all of the mid-gut is lined by head process. Whether 

 or not any of it enters into the formation of the fore- and hind-guts is not clear. 



