4 K. MIT«UKUKI. 



of the two layers in the niniiioii will Ik- easily understood. Where 

 the layer« reaeh the le\el of the _ueiieral surface of the ])lastoderin, 

 the epiblast presents a thickened ridu'e aloni;- the whole u|>|)er ed^e of 

 the ii'i'oove. As it i.s hy the _ü'rowth backward of this cdu'e that the 

 amnion comes to co\'er the endjryo, tliis ridge of the epihlast must 

 he the seat of an active i^row th. There is also in the mcthan line a 

 thickened rid_ue (Fig. 51:), (•.)oftlie epildast which starling from the 

 hottcjm of the groo\e reaches as far a.s tlic lexel of the hlasloderm, 

 litling in its ujjward course the still oj»en medullarv !jroo\ e of the 

 head of the endjryo. 



In the semicircular tI•an^])arent area in front (jf the anterior 

 liorse-shoe shaped groo\e, the epihlast consists of two layers of pave- 

 ment cells, of which the up|)er is especially Hat and seems to be of 

 stitf consistency. The hypoblast in the region directly in front of 

 the groove consists of polygonal cells. The o[)aque semicircular line 

 in the transparent area already spoken of seems to be due to a special 

 acGimiulation of the hypoblastic (Fig. 58.) cells. A little in front 

 of this line, the hyjjoblast becomes suddenly a mass of live yolk 

 granules with nuclei scattered among them. At the periphery of the 

 transpai'cnt area this passes rather abruptly into a 1)ed of large y«>lk 

 spherules. 



The fact that the amnion in Kept ilia, consists, when Hrst laid, 

 only of the two primai-y layers was made known by IStrahl (No. 5), 

 Hoffman (No. (i), Ferenyi (No. 16), and Kavn (No. 9). Hoffman 

 pointed it out as a point of great difference between the amnion <jf 

 l\e{)tilia and that of lîirds, but it is now well kno\\n lliai, in Jîirds 

 also, the amnion consists at first only of the epihlast and hypoblast. 

 Kijlliker refers to the fact in his classical work (zweite Auf. j).l.SS. and 

 Fig. 85). and Ka\n (No. 8) has woi-ked out the point ehdiorately. \ an 

 Beneden and I'h. dulin (No. 11) also ob>erved the same fact in the 



