32 BIOLCGY OF THE LABORjiTORY MOUSE 



the mesoderm, and anteriorly with the head process (which is entodermal in 

 nature and indistinguishably fused with the original proliferation of ento- 

 derm). Of these three, it gave rise to two, the mesoderm and the head 

 process. It may be added that the somewhat distinct proliferation of 

 mesoderm cells that produced the allantois occurred at its posterior end. 

 Its own cells are undifferentiated in nature and cannot be classified as either 

 ectoderm, entoderm or mesoderm. The only structure in primitive chor- 

 dates possessing these characteristics is the dorsal lip of the blastopore, and 

 it is probable that the primitive streak and the dorsal lip of the blastopore 

 are homologous. Besides being a point of origin for new tissues, it is the 

 center of a region of rapid growth. In sections it may be observed that the 

 adjacent mesoderm is full of dividing cells, and as we have seen, cells from 

 this region are continually pushed forward to give rise to somites anterior 

 to the primitive streak, so that much of the increase in length of the embryo 

 is due to growth in this region. We have already mentioned the head fold as 

 a growth center. There are thus two regions of particularly active growth 

 in the developing embryo, the primitive streak and the head fold (24). It is 

 interesting to note one point of contrast between these two; namely, that the 

 tissues in the head region are well advanced in difTerentiation while the 

 tissues of the primitive streak region remain relatively undifferentiated. 



The coelom. — Coincident with the formation of the somites in the 

 paraxial mesoderm, the coelom or body cavity develops in the lateral meso- 

 derm. It is formed by a division of this mesoderm into two layers, a dorsal 

 or somatic layer adjacent to the ectoderm, and a ventral or splanchnic layer 

 adjacent to the entoderm. The coelom is the space between the two 

 (Fig. 17). Because the somatic mesoderm and the ectoderm are closely 

 associated and undergo many foldings in common, it is convenient to desig- 

 nate the two layers together by the term somatopleure. For the same 

 reasons splanchnic mesoderm and entoderm together are designated as 

 splanchnopleure. It should be noted that the mesoderm and ectoderm of 

 the somatopleure dorsal to the coelom are continuous with the similar layers 

 in the amnion. The amnion, therefore, may also be classed as somatopleure. 

 In like manner the mesoderm and entoderm of the splanchnopleure ventral 

 to the coelom are continuous with the similar layers in a tissue which bounds 

 the extra-embryonic coelom laterally. This tissue, therefore, may also be 

 classed as splanchnopleure. 



It has been previously stated that there is a mass of mesoderm in the 

 mid-sagittal plane anterior to the fore-gut. This extends to right and left, 

 across the front of the fore-gut and is continuous laterally with the lateral 



