XIII 



PHYLrM CH(JRDATA 



I'Jl 



wall of the ea-lomo — i.e., from nics()<lerni, parti}" IVoiii the s^'inpa- 

 thetic ganglia. There may bo numerous adrenals segmen tally 

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

 produce an iutcriial sccrc/ion, ^vh'ich mingles with the ])lo(>(l .iiid 

 ]»rodufos ])hvsi()logical ert'ects on other parts. 



Development. — -The ovaot'Craniataare usually telolecithalJ)ut 

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

 small in ([uantity segmentation is complete but usually unecjual, 

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

 embryo proper is Ibrnied, 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 Crania ta a gastrula stage is formed by a 



erf or 



•^ .rich 



e-nl 



ntsd 



Fk;. 79-J.— Trans verve section of earlier (A) and later (B) embryos of Frog. c<>:l. ccelome ; cmI . i>ro- 

 longation of ecelonie into jirotovertebra ; tat. mescnteron ; nitd. ijr. medullary ^-oove ; //(«/. 

 mesoderm ; acli. notochord ; in-.v. proto vertebra ; .v//. d. segmental duct ; aoiu. somatic layer of 

 niesodei'm ; s;). c. sjiinal cord ; Sjtl. splanchnic layer of mesoderm ; yk. yolk-cclls. (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 codome 

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

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

 paired longitudinal bands (Fig. 792, A, ?HSf/.) lying one on each side 

 of the middle line, wdiere they are separated from one another b}^ 

 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 verichral plate, bounding the nervous 

 system and notochord, and a ventral j)ortion. the lateral 'plate, 



