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Mr. A. E. Shipley. On the Mesoblast [Nov. 26, 



sides of the mesenteron consist of columnar cells, in appearance very 

 like the epiblast, against the inner surface of which it is closely 

 pressed. The floor is composed of cells, which retain their yolk-like 

 characteristics for a considerable time. The mesoblast now begins to 

 appear by the differentiation of two bands of those yolk cells which 

 lie in the angles formed by the invaginated mesenteron and the 

 epiblast. The differentiation appears to take place from before back- 

 wards. The two bands of meisoblast are separated from one another 

 in the dorsal median line by the juxtaposition of the invaginated hypo- 

 blast and the epiblast. They are separated ventrally by the hypo- 

 blastic yolk cells which are in contact with the epiblast over the 

 lower two-thirds of the egg. Subsequently, but at a very much 

 later stage, the mesoblast is completed ventrally by the downgrowth 

 on each side of the mesoblastic plates. These proliferate cells at their 

 edge, which grow down between the hypoblastic yolk cells, and so 

 complete the mesoblast ventrally. The first formation of the longi- 

 tudinal band appears to take place by a differentiation of hypoblastic 

 cells in situ, and not by an invagination of cells.. 



This account of the origin of the mesoblast differs materially from 

 that given by Scott. According to his- observations, the mesoblast is 

 derived from two sources : (I) cells which are invaginated with the 

 mesenteron, these form the longitudinal bands ; (2) the outer layer 

 of hypoblastic yolk cells, which split off the remainder,, and form the 

 ventral sheet which completes the mesoblast on that side of the body. 

 Since by this time the head of the embryo has raised itself above the 

 yolk, there are no hypoblastic yolk cells in it,, and consequently its 

 mesoderm is entirely derived from the first source, whereas in the 

 trunk the dorsal mesoderm is derived from the first source, the ventral 

 from the second. 



By the time that the mesoblastic plates become' separated from the 

 yolk cells, the neural plate becomes evident in the exterior. This 

 extends from the blastopore as a low ridge,, over two- thirds of the 

 circumference of the egg. Its appearance is soon followed by the 

 separation of the anterior end from' the rest of the yolk. 



The invaginated endoderm has extended round for more than half 

 the circumference of the egg, and its most anterior portion is included 

 in the head, which is by this time distinct from' the rest of the 

 embryo. 



All this time the blastopore has been visible- at the posterior end of 

 the neural plate. It has been figured in this^ position by Schultze, 

 who gives a very complete set of figures of the embryo viewed as a 

 whole. The elongation of the embryo now proceeds so rapidly that 

 the anterior end curves round over the blastopore ; the posterior end 

 is much the largest, containing all the food yolk. 



Schultze, from observations upon the whole embryo, came to the 



