84 THE EARLY DEVELOPMENT OF MAMMALS. 



The Embryonic Ccelom. 



In the body of the embryo proper the ccelom acquires a very complicated dis- 

 position. It forms, first, a series of small cavities alongside of the medullary tube. 

 The walls of these cavities are termed the somites. A pair of somites mark out a 

 primitive segment. It forms, secondly, two large main cavities, which partially unite 

 in later stages on the ventral side of the embryo, the primitive segments lying more 

 on the dorsal side. These two large ccelom spaces constitute the splanchnocele, a 

 term which has reference to the fact that this space surrounds the splanchnic vis- 

 cera. Finally, it forms a series of so-called head-cavities, of which there are proba- 

 bly always three on each side of the head. The walls of these head-cavities in 

 part produce the muscles of the eye. We must now consider the development of 

 these divisions of the ccelom in the order indicated. 



The Primitive Segments. A segment consists of a pair of cavities symmetrically 

 placed and bounded by mesothelium. The cavities are portions of the embryonic 

 coelom. For convenience of description the term somite is applied to one of the 

 pair of structures which constitute a whole segment. The somites appear very early; 

 the first pair can be recognized in the chick after twenty to twenty-two hours' 

 incubation; in the rabbit, at the beginning of the eighth day. In both cases the 

 medullary groove is still nowhere closed. In amniote embryos, just before the first 

 segment appears, the mesoderm on either side of the axial line is considerably 

 thicker than farther away from it. We can, therefore, distinguish two zones, namely, 

 the thicker segmental zone near the axis, and the thinner, but much wider lateral 

 or parietal zone (Figs. 129, 131). The first step in the formation of the first seg- 

 ment is a loosening of the cells in the segmental zone, along a narrow transverse 

 line. In the chick this occurs about 0.14 mm. in front of the primitive streak, at 

 a time when only a portion of the medullary groove is formed. Very soon there 

 appears, close by, a second similar transverse loosening of the cells. The mesoderm 

 of the segmental zone is thus cleft twice, the mesodermic cells between the two 

 clefts constituting the first somite, which is somewhat cuboidal in form. The first 

 segment appears in what later becomes the occipital region. All further segments 

 are formed successively in a similar manner behind the first, the series growing by 

 additions caudad. The segments differ somewhat one from another in the details 

 of their development. The primitive somites, owing to their form and their prox- 

 imity to the anlage of the central nervous system, were taken by early embryologists 

 to be the beginnings of the vertebrae, and were, therefore, called the proto-vertebrcE. 

 This name is still used, although the idea upon which it was based is known to 

 be erroneous, because the primitive segments form much more than the vertebrae. 



The association in time of the development of the medullary groove and primi- 

 tive segments is important. By the formation of the groove the space between 

 the ectoderm and entoderm alongside the groove is increased, and it is this space 

 which gives the mesoderm the opportunity to grow in thickness so as to form the 

 segmental zone next to the medullary groove. 



