THE ONTOGENESIS OF VERTEBRATES 63 



becomes pinched off from its place of origin, not as a tube, 

 but as a solid rod of cells, the notochord, which forms the 

 precursor of the vertebral column. 



A third procedure, more complicated than the other two, is 

 that involved in the formation of the mesoderm. Like the 

 notochord, this arises also from the endoderm, and appears 

 typically in the form of paired, lateral pockets, the mesodermic 

 diverticula. There is reason to suppose that originally, that 

 is, in certain of the lost forms between the creeping gastrula 

 and Amphioxus, these diverticula were used as gonads, or 

 sac-like cavities, in the lining of which the germ cells were 

 developed, but in the vertebrates this function is retained by 

 but a very small portion of their surface, as will be shown 

 later. These diverticula soon separate themselves from the 

 intestinefand expand until they fill practically the entire space 

 between ectoderm and endoderm and lie in close contact to 

 one another. They thus form a series of paired cavities, the 

 metacccles, those of each side separated by transverse par- 

 titions composed of the walls of adjacent diverticula, and 

 those of the two lateral series similarly separated by longi- 

 tudinal partitions which lie in the median line above and be- 

 low the intestine. The early loss of the transverse partitions 

 converts the segmental series of lateral cavities into a single 

 pair, one for each side, while a similar reduction of the greater 

 portion of the ventral longitudinal partition throws the two 

 cavities together and forms eventually a single large metacoele 

 or body cavity, lined by the mesoderm. One layer of this 

 invests the outer body wall, the other the intestine, the parie- 

 tal and 'visceral layers respectively. The longitudinal parti- 

 tions, both dorsal and ventral, serve as suspensory ligaments 

 in the intestine and are termed mesenteries. The dorsal one 

 is retained throughout its entire extent; the ventral one disap- 

 pears posterior to the liver. It will be noticed that the meso- 

 dermic diverticula during their development have expanded 

 at the expense of the protoccele, the original cavity included 

 between ectoderm and endoderm, and thus at the completion 

 of the process the protoccele has become reduced to a com- 



