AMPHIBIA 205 



region at the apical pole is free from yolk granules. Maturation of 

 the egg occurs partly before lajdng, one polar body being given off 

 during the descent of the egg in the oviduct. The second maturation 

 division occurs after insemination. 



2. The Embryonic Period (Fig. 119). — ^Fertilization occurs while 

 the eggs are being laid, the spear-shaped spermatozoon penetrating 

 the gelatinous envelope and forcing a path through the yolk to the egg 

 nucleus. Cleavage is total or holoblastic in spite of the large accumu- 

 lation of yolk. The first and second cleavage furrows being meridional 

 and the third unequally equatorial, cutting off four micromeres from 

 the apical and four macromeres from the vegetal pole of the egg. The 

 micromeres cleave much more rapidly than the yolk-laden macro- 

 meres, resulting in the formation of a rather thick-walled but fairly 

 typical blastula, with numerous small pigmented cells above and com- 

 paratively few large unpigmented cells below. The hollow of the 

 blastula, or segmentation cavity, is much reduced in volume because 

 of the thickness of the cells at the vegetal pole. Gastrulation, while 

 not so simple as in Amphioxus, is clearly homologous with the latter. 

 The departure from the diagrammatic condition is due to the accumu- 

 lation of yolk, which prevents the typical embolic invagination of the 

 very thick layer of vegetal pole cells. The difficulty is evaded by 

 having the invagination take place at the edge of the thickened area, 

 where a flat infolding of surface cells takes place just below the edge 

 of the pigmented area, leaving a crescentic blastopore on the surface. 

 The main part of the gastrulation process is accomplished by the 

 overgrowth of the endoderm cells by the ectodermal cap, a process 

 known as epibolic gastrulation. The archenteron, at first flat and with- 

 out a lumen, soon expands and largely displaces the segmentation 

 cavity. The gastrula is morphologically a two-layered embryo, with 

 a layer of ectoderm on the outside and a layer of endoderm within, 

 though in the frog each of these layers is more than a single cell layer 

 in thickness. Mesoderm formation is accomplished by the ingrowth of 

 a sheet of cells around the blastopore. This zone-like layer soon 

 splits into two layers, an outer somatic and an inner splanchnic layer, 

 with the primitive coelomic cavity between. Coelomic pouches not 

 unlike those in Amphioxus arise from the dorsal lateral regions of the 

 archenteron, and a median dorsal strip of the latter is left over to 

 form the notochord. The development of the central nervous system 

 is decidedly precocious, for even in a late gastrula stage the medul- 



