THE EARLY EMBRYOLOGY OF THE MOUSE 31 



anterior end of the primitive streak. Each member of the pair appears as a 

 localized denser area which grades off anteriorly into loose mesoderm, and f^^cU'^^'n^ 

 which, posteriorly, is separated by a cleft from the undifferentiated caudal -'^^^<2 

 mesoderm. The second pair forms posterior to the first, and is Hkewise 

 separated by a cleft from the undifferentiated caudal mesoderm. Additional 

 pairs of somites form at more or less regular intervals, each new pair differ- 

 entiating just posterior to the pair last formed, until a total of 65 pairs* 

 has appeared (9). Continued and rapid proHferation of the mesoderm on 

 each side of the primitive streak maintains a supply of undifferentiated cells. 

 These push forward to about the level of the base of the notochord where the 

 new somites are segmented off in regular succession. As a consequence of 

 this manner of formation, the anterior somites are the oldest and, at any 

 given stage, the most highly differentiated while the posterior somites are 

 the youngest and the least differentiated. 



Figure 18 shows in interesting fashion the way in which the "zipper 

 action" by which the fore-gut is formed moves the location of the anterior 

 intestinal portal steadily^ caudad. At the two somite stage the opening of 

 the shallow fore-gut lies well ahead of the first somite. At the seven somite 

 stage the anterior portal has moved backwards until it is just about at the 

 level of the first somite. At the eleven somite stage it has reached approxi- 

 mately the level of the sixth somite. (The portion of the gut shown in 

 Fig. 18C is mid-gut just caudad to the anterior intestinal portal.) 



Because of the regularity with which successive somites develop, the 

 number of somites in an embryo is a convenient means of stating its stage 

 of development. V 



The primitive streak as a growth center. — The primitive streak is ^-^ 

 remarkable as being a region in which the three germ layers meet (Fig. 15). iQi 



It is continuous dorsally with the ectoderm, laterally and posteriorly with 



— — 



* This figure is for the rat; the characteristic number may be slightly different for 

 the mouse. There is some individual variation. Butcher states in his excellent paper 

 on the somites of the rat that the first pair dedifferentiates and disappears at about 

 the seven somite stage. We have found no evidence of such a dedifferentiation in our 

 material. As may be seen from Figs. 18A, B, and C, the first somite can be traced 

 clearly at least to the eleven somite stage. 



Fig. 18. — Sagittal sections through somites. A. Embr>'0 of 7 days 18 hours, with 

 2 somites formed. B. Embryo of 8 days i hour, 7 somites. C. Embryo of 8 days 11 

 hours, II somites. Because the embryo begins turning at about the 7 somite stage, 

 the plane at which this 11 somite embryo is cue, although sagittal to the first 6 somites, 

 is transverse to the mid-trunk region. Projection drawings (X150). 



