DIAGRAM XXVIII. 

 The Frog. 



Skull I and Skull 2 (after W. K. Parker), show the under and upper 

 aspects of the frog's skull: — e.o. ex-occipital; f.o. prootic ; /«;-. para- 

 sphenoid; sp.e. sphenethmoid ; v. vomer; pm. premaxilla ; mx. max- 

 illa ; pal. palatine ; pt. pterygoid ; q.j. quadrato-jugal ; q. quadrate ; 

 sus. suspensorium ; p.f. parieto-frontal ; n. nasal; sq. squamosal. 



Corp. (after Auerbach) shows a much magnified red blood-corpuscle : — 

 Cm. the cell-membrane; C. the cortical cell-substance; M. the me- 

 dullary cell-substance ; N. the nucleus ; Nu. the nucleoli. 



A, B, and C (from Balfour, after Ecker and Mivart), show three stages 

 in the life-history of the frog. In A, the external gills and the suctorial 

 pits is) are shown. 



Fig I (after Hertwig) shows the first segmentation of the ovum, 

 sp. the nuclear spindle, p. the pigmented pole. 



Fig 2 (after Ecker) shows segmented ovum with about 64 cells. 



Fig 3 (after Hertwig) shows a section at the gastrula stage : — ep. the 

 epiblast ; hy. the invaginated hypoblast ; arch, the archenteron ; bl. the 

 blastopore; y.c. the yolk cells; s.c. the segmentation cavity. 



Sect. I (from Balfour, after Gbtte) shows a longitudinal section of an 

 embryo frog: — m.c. the medullary canal ; n.c. the neurenteric canal; 

 bl. the blastopore ; g. the primitive gut beneath which are the yolk-cells. 

 The mesoblast is represented as solid black. 



Sect. 2 (after Hertwig) is a transver.se section : — vi.g. the medullary 

 groove; notoch. the notochord; g. the gut; m. som. the somatic meso- 

 blast; m.spl. the splanchnic mesoblast. 



