252 GENERAL ZOOLOGY 



rise to connective tissue structures. Figure 155 is a longitudinal 

 section of an early frog embryo. 



Ganglia. — Placodes are thickened areas in the inner layer 

 of the head ectoderm which give rise to sensory epithelia and to 

 varying portions of the ganglia of certain cranial nerves. Thus 

 the ganglia of cranial nerves V, VII, IX, and X are formed of 

 placodal cells as well as of cells derived from the neural crest. 

 The ganglion of the auditory nerve (VIII) is formed entirely 

 from placodal material. The ganglia of spinal nerves, as already 

 noted, are derived entirely from the neural crests. The ganglia 



mb 



Fig. 155. — Diagrammatic longitudinal section of early frog embryo, a, anus; 

 b, blastopore; e, ectoderm; en, endoderm; fb, forebrain; h, hypophysis; hb, hind- 

 brain; m (arrow), mouth; m, mesoderm; mb, midbrain; n, notochord; nc, neu- 

 renteric canal; p, pharynx; r, rectum; sc, spinal cord; y, yolk. 



of the sympathetic and parasympathetic systems are formed 

 later from cells that migrate out from the central nervous system. 

 Notochord. — The notochord develops from cells forming part 

 of the roof of the archenteron, derived by invagination from the 

 dorsal-lip region of the blastopore. These cells, at first insepa- 

 rable from those forming the lining of the archenteron, soon 

 become organized or differentiated into a rod-shaped form lying 

 directly beneath the medullary plate and, later, the neural tube. 

 This rod of cells is the rudiment of the notochord which histo- 

 logically is classified as a form of connective tissue. It becomes 

 completely separated from the endodermal cells of the arch- 

 enteron. The notochord is developed in all animals belonging 

 to the phylum Chordata, which includes the subphyla Enterop- 

 neusta, Tunicata, Cephalochorda, and Vertebrata. The noto- 

 chord evolved apparently as a skeletal axis and serves as such 



