xm 



PHYLUM CHORDATA 



117 



found certain " ductless glands," the adrenals or inter- and supra- 

 renal bodies. They are developed partly from ridges of the dorsal 

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

 pathetic 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 ova of 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. 



A metier 



enl 



msd- 



msd 



FIG. 802. Transverse section of earlier (^1) and later (B) embryos of Prog. cod. coelome~;><r. 

 prolongation of coelome into proto vertebra ; ent. mesenteron ; med. gr. medullary groove; 

 msd. mesoderm ; nch. notochord ; pr. v. protovertebra ; sg. d. segmental duct ; som. somatic 

 layer of mesoderm ; sp. c. spinal cord ; spl. splanchnic layer of mesoderm ; yk. yolk-cells. 

 (After Marshall.) 



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

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

 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 coelome 

 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. 802, 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 (nch.), 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 



