THE EARLY EMBRYOLOGY OF THE MOUSE 



29 



occurs anterior to the fore-gut (Fig. 15). The primitive streak in the mid- 

 sagittal plane consists of a tissue which joins, and in structure is intermediate 

 between, ectoderm and mesoderm. Whether or not this should be called 

 mesoderm is a matter of definition. At the caudal extremity of the embryo 

 its structure is essentially that of true mesoderm, and it may accurately be 

 said that there is mesoderm in the mid-sagittal plane in this region. 



.eVo*^ 



Fig. 17. — Section transverse to mid-trunk region of 8 day, 4 somite embryo. Through 

 2nd somite. Projection drawing (X150). 



While the notochord blocks the entrance of mesoderm into most of the 

 mid-sagittal area, there are well developed sheets of mesoderm on each side 

 of this area. These lie between ectoderm and entoderm and are continuous 

 laterally with the extra-embryonic mesoderm (Fig. 17). Anterior to the 

 primitive streak, it is convenient to recognize two distinct areas in these 

 mesodermal sheets, an area of paraxial mesoderm adjacent to the notochord, 

 and an area of lateral mesoderm adjacent to the extra-embryonic coelom. 

 The former gives rise to the somites, the latter to the mesoderm of the 

 embryonic coelom. At 7I2 days there is no visible division between 

 the two areas (Fig. 19B), but beginning at about 7^4 days, coincident with 

 the development of the somites, they are separated by a longitudinal cleft 

 that becomes increasingly pronounced as the differentiation of the somites 

 progresses (Fig. 17). 



The somites are paired, segmental structures arising in the paraxial 

 mesoderm (Figs. 18 and 25D). They are the first indication of metamerism 

 in the developing embryo. The first pair forms a little anterior to the 



rated by any mesoderm, 

 the rabbit. 



This area is probably homologous with the proamnion of 



