in the Foetus of Vertehrated Animals. 209 



of the bird is best l^nown, Ave shall begin with it, and shall after- 

 wards notice the analogous observations which have been made 

 in other vertebrated animals. 



1. Changes of the Germinal Membrane in Birds. 



In the unincubated egg of the common fowl, the cicatricula, 

 or germ spot, lying on the surface of the yolk, is of a round 

 form, a whitish colour, and generally about one-sixth of an inch 

 in diameter (Plate II. Fig. 1). The disk of the cicatricula is form- 

 ed of different-sized granules united together, and is.covered by 

 the proper membrane of the yolk, (Fig. 3). 



The central part of the disk (Figs. 2. and 3, a) is thinner and 

 more transparent than the rest, and has been called the CoUiqua- 

 mentum, or Transparent Area. Observers differ as to whether 

 any mark or trace of the embryo is to be found in the cicatri- 

 cula before incubation has commenced : Some, as Rolando, 

 Prevost, and Dumas, affirm that there always exists a linear 

 trace of the embryo in the centre of the transparent area (Fig. 4. 

 b) ; while others, as Pander and Baer, assert that they have 

 never been able to discover such an appearance. However 

 this may be, it seems now to be established, that after the egg 

 has been incubated seven or eight hours, a small dark line may, 

 with the aid of a magnifying lens, be discovered on the upper 

 part of the cicatricula, towards the centre of the transparent area 

 (Fig. 4. b). 



This line, or primitive trace, is swollen at one extremity, and 

 is placed in the direction of the transverse axis of the egg ; its 

 rounded extremity is situated towards the left, when the small 

 end of the egg is turned from us, and indicates the place where 

 the head of the foetus is afterwards formed (Figs. 1. and 4). This 

 large extremity occupies very nearly the centre of the transpa- 

 rent area, while the linear part of the primitive trace (Fig. 5, b), 

 corresponding to the body and tail of the foetus, approaches the 

 margin of that area on the right side. 



As incubation proceeds, the whole cicatricula, expanding, in- 

 creases in size. The transparent area becomes larger, more pel- 

 lucid and defined. We are indebted to Pander for the import- 

 ant discovery, that, towards the 12th or 14th hour, the germi- 

 nal membrane becomes divided into two layers of granules — the 



u 2 



