102 



EMBRYOLOGY. 



90S 



erfg _6d 2 



Two germ-layers first appear in 

 whicli liave 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 embryonaHs, or as embryonic spot It 

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

 are separated by a distiact fissure, and may 

 be detached from each other. The inner 

 germ-layer (ik) 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 Eauber, and which 

 have been named after him Eauber's layer. 

 Toward the margins of the embryonic spot 

 the outer layer becomes thinner and pos- 

 sesses only a siugle 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 

 blastula 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 amaboid 

 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 

 {hy) 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 

 (bp). I, on the contrary, believe that the gastmlation takes place in the 

 manner described on page 104. 



