PROCESS OF DEVELOPMENT IN AVIAN OVA. 517 



so-called " tread," or cicatricula, constitutes the germ, which is 

 enclosed in the same general investment as the yellow yolk. 

 Pander * described the cicatricula as composed of two easily 

 separable layers, of which one dips into the yellow yolk, whilst 

 the other forms a layer upon its surface. The latter, he says, 

 is a round disk, in and from which the foetus is formed, and 

 which may therefore justly lay claim to the title of blastoderm 

 (germ-membrane, Keimhaut). The former part was termed by 

 Pander the nucleus of the cicatricula. This is a constituent of 

 the so-called white yolk, which lies beneath, but is not con- 

 nected with, the central transparent part of the germ-membrane. 



B 



Fig. 402. F N a P Z, as in the preceding figures ; kk, sections of. 

 the annular swelling and margins of the anus. The- punctated line 

 between k P, indicates the antecedent connection of the yolk plug 

 with the germ-cell mass, z.. The ovum in this condition has already 

 performed the rotation, the dorsum lying above, the abdomen below. 



The statement of Pander, that the embryo is developed 

 exclusively from the germ disk, remains unshaken up to this 

 time. Keichertf and His:}: have certainly endeavoured to show, 

 from various points of view, that the morphological elements 

 of the white yolk enter into the new animal body. His, on 

 the ground of his own observations, has also designated the 

 germ disk as the principal germ (Archiblast or Neuroblast), 

 and the accompanying constituent of the white yolk as the 



* Beitrcige zur Entwickelungsgeschichte des Huhnchens, (Essays upon the 

 history of the development of the Fowl.) Wiirzburg, 1817. 

 t Entwickelungsleben im Wirbelthierereiche, 1840. 

 Untersuchungen iiber die erste Anlage des Wirbelthierkibes, 1868. 



