304 Dr Allen Thomson on the Vascular System 



of whom have investigated more particularly the changes in the 

 germinal membrane subsequent to the period when the parts of 

 the fcetus begin to be visible, the first trace of the embryo appears 

 like a dark line near the middle of the central transparent part, 

 exactly in the same manner as the primitive trace shows itself in 

 the transparent area of birds (Fig. IV. b). In comparing, in- 

 deed, the drawings which these observers have given of the de- 

 velopment of the rudimentary parts of the foetus of the dog and 

 rabbit, with those at a corresponding period in the eggs of the 

 common fowl or duck, we cannot fail immediately to perceive 

 the singular resemblance which exists between them. 



The spinal canal and cerebral cavity, the vertebrae, the inte- 

 guments, and the amnios, are most probably, therefore, formed, 

 as in the chick, by the extension and folding of the serous layer 

 of the germinal membrane. The intestine takes its origin in the 

 same way as in the chick, by the inflection of the mucous and 

 vascular layers on the anterior or abdominal side of the embryo 

 (Fig. 8. g g), and is there connected in a similar manner with the 

 sac of the yolk. In the dog and rabbit the peripheral part of 

 the germinal membrane undergoes changes also very similar to 

 those described in the chick : there is formed on its surface a 

 vascular network which receives its blood from the omphalo- 

 mesenteric arteries ; this network is smaller, relatively to the 

 size of the foetus, in these animals than in the bird, and its ves- 

 sels are more numerous (Figs. 7, 8. k Ic). Unfortunately, the 

 first appearance of the heart has not been investigated. 



3. Changes in the Germinal Membrane in Reptiles. 



The ova of the Saurian and Ophidian reptiles resemble much 

 more than those of the Batrachian, the eggs of birds. Baer has 

 represented the cicalricula or germinal disk in the ova of the 

 Coluber natrix, the Lacerta agilis, and the L. crocea ; and he 

 informs us that he has ascertained by observation, that the 

 changes which this part undergoes during the development of 

 the embryo in these animals, do not diflPer in any important 

 points from those which takes place in the bird. 



The ova of Batrachian reptiles differ verv much in structure 

 and appearance from those of the Lizard and Serpent tribe, as 

 well as from those of birds and mammalia. In the ova or spawn 



