208 GENERATION. 



arise from the membrane round the head ; and, as it increases, it 

 gradually covers the upper or exposed surface of the head, like a hood ; 

 then gradually extends itself along the body, covering more and more 

 of it towards the tail, having always a determined edge : and when 

 got to the tail, it there closes up the animal entirely, on the upper side, 

 and which has only the membrana vitelli upon it, making a circum- 

 scribed cavity, in which the chick lies, and which I call the ' amnios,' 

 as being the immediate covering of the chick, composing a part of the 

 secondines or after-birth. 



This membrana vitelli [germinal membrane] would appear to have 

 formed itself from the intestine ; if so, then it was prior to that part 

 being visible ; or it might be considered an expansion of, or a process 

 from, the intestine over the yolk, and under its own proper membrane. 

 That part next to the chick appears to divide into several laminae, or 

 has the power of forming several ; for we find, by the time the whole 



has formed such and such j>arts, that we can separate it into 



laminae, which are seen in Plate , figs. \ This membrane is 



extending itself over the yolk, expanding itself till its edges come beyond 

 the largest diameter, and now, as it expands in length from the chick, 

 it contracts at its edge, and at last encloses the whole yolk, forming on 

 the opposite side something like a cicatrix, to which the last part of 

 the slime adheres. 



From this account of the yolk, and this membrane, it might appear 

 that this membrane was only at first a covering communicating with 

 the belly of the chick, preparatoiy to, or for the entrance of the yolk 

 into the abdomen just before hatching. But from its structure it would 

 appear to have some use while under incubation, for it first becomes 

 extremely vascular, and on its inside it is thrown into ruga? 2 , as if an 

 increase of inner surface was necessary : wherever this membrane 

 advances, the yolk becomes fluid, beginning at first where the mem- 



framer or framework of the organs of the animal functions of the chick, seems 

 evident from its extending beyond those parts, over the yolk, to form the false 

 amnios; it only forms the cuticle and the amnios of the embryo itself. It is because 

 the membrane is folded over the substanee of the medulla spinalis and vertebra?, as 

 these are formed, that it has been said to form them. It was first described by 

 Pander, in Ids masterly Thesis entitled " Dissertatio sistens historian! metamor- 

 phoseos, quam ovum incubatum prioribus quinque diebus subit." 8vo. 1817.] 



1 [See Physiological Catalogue, 4to. vol. v. fig. 7. plate 69, fig. 5. plate 70, 

 and the beautiful magnified view of the chick resting upon the yolk, in plate 71, 

 where b, the serous layer, is reflected from d, the vascular layer and mucous 

 layers of the germinal membrane, or ' membrana vitelli ' of Hunter. See also the 

 mucous layer, /, fig. 5, plate 75. reflected from g, the vascular layer of the germinal 

 membrane or vitelline sac, in a further-developed embryo.] 



' 2 ['Yasalutea' of Haller.] 



