ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 761 



germ is used up. He derives his mesoderm from large spherical 

 elements, described by him as cleavage-spheres, which wander from 

 the floor of the germinal cavity into the space between the primitive 

 layers. These elements, the megaspheres of His, are when first 

 evident merely the residual cells of the germ ; at a later stage they 

 are aggregates of cells whose nuclei and borders have become incon- 

 spicuous through inception of yolk-granules. They are endowed 

 with an extraordinary power of multiplication. The inner lamina, 

 once formed, shuts off the rest of the germ from the germinal cavity, 

 and by this time has received into itself the sjmeres in question, now 

 for the most part resolved into their constituents, whose upward 

 movement from the floor of the germinal cavity is due to their being 

 specifically lighter than its contained fluid. 



His resolves the germ of the new laid egg into one lamina, the 

 outer, designating the residual cells collectively by the not well 

 chosen name of subgerminal processes. After the formation of the 

 inner lamina there appears in connection with the primitive streak 

 what he justly enough terms the intermediate stratum. His does 

 not derive all the connective tissues from this stratum, but imagines 

 a migration of cells from the white yolk, out of which the vascular 

 system is developed. 



Until the appearance of the primitive streak the germ grows 

 uniformly throughout. At its margin the outer and inner laminae 

 make together an acute angle, between the legs of which, as seen in 

 sections, cells of the middle germ are packed. As soon as the 

 primitive streak is formed, these cells back towards it and become 

 involved in the constitution of the axial plate. Thereupon the 

 margin, deprived of its intermediate cells, appears as a solid keel, with 

 three or four tiers of cells only, exclusively made up of the ectoderm 

 and endoderm. While this peripheral region continues to spread 

 itself, the under cells separate from the upper, of which only a single 

 tier now remains in continuity with the outer lamina. Some of the 

 separated cells at once assume the features of the cells of the inner 

 lamina (roofing the germinal cavity), as a prolongation of which they 

 extend, beneath the outer layer, under the form of a thin membrane. 

 This limiting membrane rests on the white yolk, but does not attain 

 the extreme periphery of the germ. It there passes into an irregular 

 mass composed of the other separated cells, which wander into the 

 white yolk, become stellately branched, and anastomose to form a 

 network whose meshes get filled with white yolk-spheres. Close to 

 the margin of the germinal cavity, disappearance of the yolk-spheres 

 changes this reticulum into a solid (polyderic) layer, passing gradually 

 into the monoderic inner lamina. It can scarcely be doubted that all 

 the cells between the outer lamina and the white yolk must be con- 

 sidered as representing the endoderm peripherally. The reception of 

 granules from the white yolk may be termed a sort of primitive 

 digestion. Subsequently, when the vascular lamina grows into the 

 area opaca, it is separated from the white yolk by the limiting 

 (endodermic) membrane, and its first formed vessels take up the 

 nutrient fluid which through this membrane they receive. Cells, 



