114 



ZOOLOGY 



SECT. 



certain "ductless glands," the adrenals or supra-renal bodies. They 

 are developed partly from ridges of the dorsal wall of the coeloine- 

 i.e., from mesoderm, partly from the sympathetic ganglia. There 

 may be numerous adrenals segmen tally arranged, or a single pair. 

 Their function is quite unknown, but their abundant blood-supply 

 points to their possessing a high physiological importance. 



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. 



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

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



ncJt 



e-nt 



msd 



B 



pr.v 



msd 



FIG. 748. Transverse section of earlier (A) and later (B) embryos of Frog. cal. ccelome ; col', pro- 

 longation of ccelome into protovertebra ; ent. mesenteron ; mal. gr. medullary groove; //<.>/. 

 mesoderm; m-k. notochord ; pro. protovertebra ; g. d. segmen tal duct; sow. somatic layer of 

 mesoderm ; sp. c. spinal cord ; spl. splanchnic layer of mesoderm ; ?/A-. yolk cells. (After 

 Marshall.) 







combination of in-pushing and over-growth : the 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. 748, A, mscL) lying one on each side 

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

 the medullary tube (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 



