FORMATION OF THE EMBRYO IN THE FOWL'S EGG. 635 



composed of uniformly rounded cells agglutinated with each other, as 

 shown in Figs. 184, 185, 186. 



During the third day the inner and lower portions of the protover- 

 tebrae extend toward the median line in such a manner as to surround 

 the chorda dorsalis, above, below, and on each side, with an investment 

 of new material. As these newly formed portions of the protovertebrae 

 coalesce with each other, the chorda dorsalis becomes covered with a 

 continuous tubular sheath, or investing membrane. This forms a rudi- 

 mentary vertebral column ; since the substance of the sheath, in the 

 further progress of growth, becomes first cartilaginous and afterward 

 bony, producing finally the bodies of the vertebrae. 



While the sheath of the chorda dorsalis is thus formed from the 

 inner and lower portions of the protovertebrae, their inner and upper 

 portions extend, as a thin expansion on each side, between the medul- 

 lary canal and the tegumentary layer of the ectoderm. On the fourth 

 day these lateral growths meet and coalesce at the median line, on the 

 dorsal aspect of the embryo ; and the embryonic spinal cord is then 

 enclosed in a membranous investment, similar to that of the chorda 

 dorsalis. In this investment there are afterward formed the oblique 

 processes of the vertebra?, which, by uniting with each other in the 

 same way on the median line, finally enclose the cerebro-spinal axis in 

 a series of vertebral arches. The deposit of cartilaginous matter, in the 

 double membranous tube thus formed, takes place at successive points 

 in a linear series ; and from each point the cartilaginous deposit, as 

 well as its subsequent ossification, extends gradually to its final limit. 

 The intervening portions, not converted into cartilaginous and bony 

 tissues, become, between the bodies of the vertebra?, the intervertcbral 

 ligaments, and between the dorsal arches the yellow ligaments of the 

 vertebrae. 



But in this transformation of a portion of the protovertebrse into 

 a permanent spinal column, the final arrangement of the parts is differ- 

 ent from that at the beginning. When the cartilages of the permanent 

 vertebra? make their appearance, they do not correspond in situation 

 with the original protovertebra?. Subsequently to the fusion of the 

 protovertebra? with each other, a new segmentation takes place, the 

 lines of division passing through the former intervening spaces. Each 

 permanent vertebra therefore corresponds in position with the adjacent 

 halves of two protovertebra? ; and the middle portion of each protover- 

 tebra is finally replaced by an intervertebral ligament. 



Area Tr/sci'losa, Blood and Blood-vessels. The mesoderm, during 

 the earliest periods of incubation, is less rapid in its lateral extension 

 than the two other blastodermic layers ; but it undergoes important 

 changes in texture, which lead to the development of the vascular sys- 

 tem. This begins in the second day. Within the body of the embryo 

 at this time the mesoderm exhibits on each side, at some distance 

 from the median line, a horizontal cleft (Fig. 185, p), by which it is 

 divided into two lamina?, one above, contiguous to the ectoderm, and 



