DEVELOPMENT OP THE TWO MIDDLE GERM-LAYEES. 135 



but in which no traces of the .medullary folds are to be observed. 

 The outer germ-layer (ak) is composed of a single layer of tall 

 cylindrical cells, the inner germ-layer (ik) of a single sheet of 

 greatly flattened elements. In the space between the two there 

 penetrates at both sides of the primitive groove a mass of small cells- 

 in many superposed layers, the middle germ-layer (mh). In the 

 region of the primitive groove (pr) this goes over continuously into 

 the outer germ-layer, the cells of which are here found in prolifera- 

 tion, whereas its lateral wings are separated from the outer layer by 

 a fissure. The lower germ-layer is drawn by Koller— from whose 

 work the accompanying figure is taken — as being everywhere a 



Fig. 97. — Cross section through the embryonic area of a Hole, 'which is in a stage correspondlngr 



approximately with that of the Rabbit represented in Pig 89 B, 'After Heape. 

 The section passes through the primitive groove, somewhat behind the one represented in Fig. 94. 

 ak, Outer, ijfc, inner, mk, middle germ-layer ; u, primitive groove. 



separate sheet of flattened cells. It is clear, however, from other 

 drawings and descriptions by Duval, Eabl, and others, as well as 

 from the accounts in regard to the similar development of EeptUes, 

 that for a certain distance underneath the primitive groove the 

 middle germ-layer is as little to be distinguished as a separate 

 structure- from the lower as it is fi^om the upper germ-layer. 



Cross sections through the primitive groove of mammalian 

 embryos are very instructive (fig. 97). According to Heape's inves- 

 tigations on the Mole, the groove (m) cuts deeply into a mass of 

 small cells. At this place all three layers are fused together ; and 

 it is only laterally to this that they are separated by means of 

 a distinct fissure, and that each is distinguishable by its character- 

 istic kind of cells — the outer {ak) by its tall, the inner (ik) by its 

 much-flattened, and the middle (mk) by its small, more spherical 

 or polygonal cells. 



The conditions of the germ-disc of the Rabbit found by van 

 Beneden are especially distinct (fig. 98). At the deep incision 



