DKYXLOPMXV1 <>F -iim: T\\<> MIIMH.K <.KKM LAY] 133 



\m[ in which 11. traces of the medullary folds are to be ol.-.-rv. <1. 

 Tin- mit.-r <_'. Tin layer (<//.) i- OOmpOted "f a Single lay.-r ..t' tall 

 cylindrical cells, th* 1 inner i:'-nii-l:iyT (Ik) of a .-ii i_ r le .-li- 



ly ilattenrd element-. In tin- space between the two there 

 priH tratrs at both .-ides of the primitive groove a mass of small IN 

 in many sup.-rpost d layer-. the in: ///,). In the 



region of tin- primitive groove (j>r) tin'- ^ms uv-r cunt iiun>u>ly 

 tin- outiT .<: -nil-layer, the cells of u-hirh ar.- In r<- fi>uinl in pmli- 

 tioii. win r as its lateral wings are separat- 1 from the outer layer }>y 

 ;i tiur.-. The lower ;.-i in lay r is drawn \>y KI.LKII fr..m frl 

 work tin- accompanying iigure is taken a> \.ry\\here a 



ak 





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

 approximately with that of the Rabbit represented in Fig 89 ,'i. After HIM,. 



; asses through the primitive groove, somewhat behind the one represented in 1 

 at, Outer, ile, inner, ink, middle germ-lax er : <-, priinkivt- i^-oove. 



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

 drawings and descriptions by DUVAL, RABL, and others, as well as 

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

 that for a en-tain distance und rmath the primitive groove the 

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

 structure from the lower as it is from the upp;-r g.-rm-Iay. p, 



Cross sections through the primitive groove of mammalian 

 embryos are very in>triu'ti\t> (tig. 97). According to HEAPE'S ii. 

 tigations on the Mole, the groove (u) cuts divply into a mass of 

 small cells. At this place all three layer- are hi- d t ir tln-r : and 

 it is only laterally to this that they are -.parat-d !>y means of 

 a distinct fissure, and that each is distinguishable by ite charact-r- 

 i-tk- kind of cells the outer (ak) by its tall, tin- innt-r (/X) hy its 

 nnu-h-flatt. md. and the middle (mk) by its small, more spherical 

 or polygonal cells. 



The conditions of the gorm-di-r of the Rabbit found by VAN 

 BENEDEN are especially di-tinct (tig. 98). At the deep incision 



