XIII 



PHYLUM CHORDATA 



121 



wall of the coelome — i.e., from mesoderm, partly from the sympa- 

 thetic ganglia. There may be numerous adrenals segmentally 

 arranged, or a single pair. Like other ductless glands the adrenals 

 produce an internal secretion, which mingles with the blood and 

 produces physiological effects on other parts. 



Development.— The ovaof Craniata are usually telolecithal, but 

 the amount of food-yolk varies within wide limits. When it is 

 small in quantity segmentation is complete but usually unequal, 

 when abundant, incomplete and discoidal. In the latter case the 

 embryo proper is formed, as in Cephalopods, from a comparatively 

 small portion of the oosperm, the rest giving rise to a large 

 yolk-sac. 



There is never a typical invaginate gastrula, as in Ampliioxus, 

 but in some of the lower Craniata a gastrula stage is formed by a 



enl 



jnad- 



FlG. 792.— Transvorve section of earlier (A) and later (B) emliryos of Prog. ml. coilome ; c«i'. pro- 

 longation of cceloine into protovertebra ; ml. mesenteron ; ned. r/r. medullary groove ; ui-id. 

 mesoderm ; ndi. uotochord ; jir.v. protovertebra ; st/. d. segmental duct ; som. somatic layer of 

 mesoderm ; sp. c. spinal cord ; sjil. splanchnic layer of mesoderm ; yk. yolk-cells. (After 

 Marshall.) 



combination of inpushing and overgrowth: details will be given 

 in the sections on the various groups. In the higher forms a 

 gastrula cannot be recognised with absolute certainty. 



The mode of development of the mesoderm and of the ccelome 

 differs strikingly from the process we are familiar with in Amphi- 

 oxus. At an early stage the mesoderm is found in the form of 

 paired longitudinal bands (Fig. 792, A, msd.) lying one on each side 

 of the middle line, where they are separated from one another by 

 the medullary groove (md. gr.) and the notochord {oicJi.), and com- 

 pletely filling the space between the ectoderm and the endoderm. 

 In all probability the mesoderm is derived from both of the primi- 

 tive germ-layers. Each mesoderm-band becomes differentiated 

 into a dorsal portion, the vertebral plate, bounding the nervous 

 system and notochord, and a ventral portion, the lateral plate, 



