102 



EMBRYOLOGY. 



Two germ-layers first appear in eggs 

 which have already attained a diameter of 

 more than 1 mm. and are about five days 

 old. At the place where the cell plate pre- 

 viously lay, one sees by inspection from the 

 surface a whitish spot, which is at first 

 round, but later becomes oval or pear shaped. 

 It is generally designated at this stage as 

 area embryonalis, or as embryonic spot. It 

 consists of two germ-layers (fig. 60), which 

 are separated by a distinct fissure, and may 

 be detached from each other. The inner 

 germ-layer (ify is a single sheet of greatly 

 flattened cells. The outer germ-layer (ak), 

 on the contrary, is considerably thicker, and 

 shows that it is composed of two sheets of 

 cells : (1) a deeper layer of cubical or round- 

 ish, larger elements, and (2) a superficial 

 layer of isolated flatter cells, which were first 

 accurately described by RAUBER, and which 

 have been named after him RAUBER'S layer. 

 Toward the margins of the embryonic spot 

 the outer layer becomes thinner and pos- 

 sesses only a single layer of cells ; these are 

 continuous with the large flattened elements 

 which, as we have seen, alone constitute the 

 greater part of the wall of the sac in the 

 blast ula stage. The inner germ-layer is 

 at first developed on only a small part of 

 the wall of the sac at the embryonic spot 

 and its immediate vicinity; it terminates 

 with a free notched margin, where there 

 are to be found loosely associated amoeboid 

 cells, which by their increase in number and 

 migration probably cause the further growth 



end of the first stages of cleavage. He interprets in the originally solid 

 sphere of cells (fig. 58 A) the darker and larger centrally located elements 

 (Jiy) as entoderm, the layer of smaller and clearer cells (ep) surrounding the 

 latter as ectoderm, and a small vacuity in this investing layer as the blastopore 

 (&p). I, on the contrary, believe that the gastrulation takes place in the 

 manner described on page 104. 



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