952 OF GENERATION EMBRYONIC DEVELOPMENT. 



cranium as far as the space between the auditory capsules, or even to that 

 point which subsequently becomes the pituitary fossa. The chorda dorsalis 

 is originally composed of nucleated cells, that in the first instance lie in 

 contact with one another, but are soon separated by the development of 

 intercellular substance or matrix, the whole being inclosed by a delicate 

 sheath. The chorda dorsalis, though permanent in the lowest fishes, is only 

 a transitory structure in Man and all the higher vertebrata. Soon after 

 the laying down of this cartilaginous rod, the portion of the middle lamina 

 which immediately surrounds it (uivp, Fig. 352, mv, Fig. 353) becomes de- 

 tached ; and in the portion so separated the first rudiments of the vertebne 

 appear in the form of rings surrounding the chorda dorsalis. More ex- 

 ternally, the middle layer, passing outwards and downwards, and forming 

 what is known as the Ventral Lamina, subdivides into two by a split or 

 fissure GJJO), which is the first indication of the pleuroperitoneal cavity, the 

 external or parietal layer (hpl) joining with the epiblast to form the soma- 

 topleure or walls of the abdomen with their contained structures, and ulti- 

 mately meeting on the median line and inclosing the abdominal cavity in 

 the same way that the dorsal laminie inclose the spinal cord, whilst the inner 

 layer, uniting with the hypoblast (df) develops the muscular and fibrous 

 layers of the intestinal canal, the innermost layer or hypoblast (dd) pro- 

 ducing the proper epithelium of the ducts and portions of the parenchyma 

 of the several glands connected with the intestine. At this period the prim- 

 itive aortse (ao) and the first urinary organs or Wolffian bodies (uivh) begin 

 to be formed. These, as just stated, are developed in the material situated 

 externally to the chorda dorsalis, constituting a portion of the middle lamina 

 of the blastodermic vesicle (iiw, Fig. 353), which now becomes separate and 



FIG. 353. 



Transverse section made through the Embryo of a Fowl at the thirty-sixth hour of incubation, 

 < 100. ch, chorda dorsalis ; h, external lamina ; mr, medullary tube (spinal cord) ; (hi, intestinal gland- 

 ular layer; aw, prevertebral mass; auh, incipient, cavity in the same; sp, split in the middle lamina, 

 dividing it into an external or parietal lamina, lijil, and an internal or fibrous layer, df, which are con- 

 nected by a median lamina, mp. The position of the Wolffian bodies is represented by ung, and of the 

 primitive aorta by ao. 



distinct from the remainder. Soon after this period the prevertebral masses 

 (uw) extend themselves so as to embrace the chorda dorsalis below and the 

 spinal cord above ; the part where the two sides unite above being called by 

 Rathke the membrana reuniens superior, or membranous vertebral arch. A 

 portion of the prevertebral mass also insinuates itself between the chorda 

 dorsalis and the spinal cord, so that the former is entirely invested by it, 

 and a complete vertebral column is formed, though still only membranous, 

 presenting two tubes, the one posterior, formed by the arches, and inclosing 

 the spinal cord, the other anterior, investing the chorda dorsalis. In the 

 membranous arches a histological differentiation takes place, by which, on 

 the other hand, the cartilaginous arches are developed, and on the other, 

 the anterior and posterior roots of the spinal nerves (v, g, Fig. 338). 



7!) The formation of cartilage constitutes the second stage of the devel- 

 opment of the vertebral column. It commences about the sixth or seventh 

 week of foetal life, and rapidly extends over the bodies of the future ver- 



