THE EMBRYONIC CCELOM. 79 



of the embryo and expands into the extra-embryonic coelom or space between the 

 yolk-sac and the chorion. 



The Embryonic Coelom. 



In the body of the embryo proper the coelom acquires a very complicated 

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

 tube. The walls of these cavities are termed the primitive segments. It forms 

 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 viscera. Finally, it forms a series 

 of so-called head-cavities, of which there are probably 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 coelom in 

 the order indicated. 



The Primitive Segments.— K segment consists of a pair of cavities symmetri- 

 cally placed and bounded by mesothelium. The cavities are portions of the em- 

 bryonic ccelom. For convenience of description the term segment is usually 

 applied to one of the pair of structures which constitute a whole segment. The 

 primitive segments appear very early; the first pair can be recognized in the 

 chick after twenty to twenty- two hours' incubation; in the rabbit, at the begin- 

 ning of the eighth day. In both cases the medullary groove is still nowhere 

 closed. In amniote embryos, just before the first segment appears, the meso- 

 derm on either side of the axial line is considerably thicker than further away 

 from it. We can, therefore, distinguish two zones — namely, the thicker seg- 

 mental zone near the axis, and the thinner, but much wider lateral or parietal 

 zone. The first step in the formation of the first segment 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 seg- 

 mental zone is thus cleft twice, the mesodermic cells between the two clefts 

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

 segment appears in what later becomes the occipital region. Two or, according 

 to some authorities, three segments are formed on the cephalic side of the first 

 segment; and, meanwhile, the number of segments is also increasing on the 

 caudal side of the first, but much more rapidly. The primitive segments, 

 owing to their form and their proximity to the anlage of the central nervous 

 system, were taken by early embryologists to be the beginnings of the vertebrae, 



