THE EARLY EMBRYOLOGY OF THE MOUSE 21 



If the reader now will refer back to Fig. 10, he will see that the entoderm 

 over the ventral extremity of the egg cylinder is stretched and the cells 

 flattened, but that near the upper margin of the embr>'onic portion of the 

 cylinder there is a sudden change to a higher type of cell. The transition is 

 particularly abrupt at the anterior margin of the cylinder. The thin or 

 flattened entoderm we shall refer to as squamous entoderm, the thick ento- 

 derm as columnar entoderm, the line of junction between the two as the 

 transition line. The reader should take time at this point to note, in Figs. 10 

 and 12, the precise location of the transition Hne. 



The limits of the head process are as follows. Caudad, it begins at the 

 anterior extremity of the primitive streak, that is to say just a little above 

 and caudad to the ventral tip of the egg cylinder. Cephalad, it extends to 

 the transition line. Laterad, at its broadest point it may extend almost 

 around the anterior half of the circumference of the egg cylinder (Fig. 14A), 

 but mostly it is narrower than this, fllling perhaps the anterior fifth of the 

 egg cylinder's circumference. 



When its forward growth brings it to the transition line, the head process 

 fuses with the columnar entoderm with which it has thus newly come in 

 contact (Fig. 13). The fusion is so complete that in later stages the line of 

 junction is completely lost. Laterally, its outer margins fuse with the 

 squamous entoderm. Meantime the squamous entoderm underlying the 

 head process, already ven.- thin, becomes increasingly attenuated, its nuclei 

 become widely separated and ver}- flat, and the cytoplasm largely disappears 

 (Figs. 14A and B). At yf^ days no further trace of it remains. 



In the course of the upward and laterad growth of the head process and 

 the forward growth of the mesoderm the two cefl layers come in contact and 

 overlap (Fig. 14A). In the regions of overlapping, the head process stays 

 adjacent to and advances over the surface of the entoderm, while the meso- 

 derm remains next to the ectoderm. At 7^2 days the development of the 

 mesoderm has brought it between ectoderm and head process ever}-w^here 

 except for a strip along the mid-sagittal plane of the embryo. As we shall 

 see later, the head process of this mid-sagittal strip gives rise to notochord, 

 while the remainder of the head process contributes to the lining of the gut. 



section shown in Fig. 12 there are several cells at the anterior limit of head process 

 growth that cannot be classified definitely as either head process or entoderm. The 

 division in the drawing in this region is partly arbitrary. When head process and 

 mesoderm come into contact there is also possibility for confusion. However, in well 

 fixed preparations cut at a favorable angle, the division in this case can almost always 

 be precisely determined. 



