302 Dr Allen Thomson on the Vascular St/stem 



than the adjacent parts, and is soon studded with numerous ir- 

 regular points and marks, of a dark yellow colour : as incubation 

 proceeds, these points become more apparent, and are gradually 

 elongated into small lines, which are united together, first in 

 small groups, and then into one net-work, so as to form what is 

 termed the Vascular Area. 



The space occupied by this network is cordiform, and is sur- 

 rounded by a vessel, gradually developed in the same manner as 

 those of the rest of the area. The newly formed vessels of the 

 space become more and more distinct as incubation advances, and 

 the orange-coloured fluid they contain acquires a darker hue : 

 the small branches of the network arrange themselves like the 

 fibrils of a leaf, on each side of the embryo (Fig. 16, k), and 

 terminate towards the embryo, in two vessels issuing from its 

 sides, which are the omphalo-mesenteric arteries (Fig. 16, I.) 

 Towards the circumference of the area, the smaller ramifica- 

 tions of these vessels open into the sinus or Vena terminalis 

 which bounds the space. 



This description of the changes which the germinal mem- 

 brane undergoes in the common fowl, may be applied to most 

 other birds, as no material difference has been perceived in any 

 • of their leading features, in the eggs of the different birds in 

 which they have been observed. 



2. Changes of the Germinal Membrane in Mammalia. 



The very small size of the ova of Mammalia has rendered it 

 difficult for physiologists at all times to observe the early signs 

 of development in these animals. De Graaf, Cruikshanks, 

 Haighton, and others, have described very small ova in the ovi- 

 ducts and cornua of the uterus of the rabbit ; but little respect- 

 ing the internal structure of the ovum is to be learned from the 

 observations of these authors, before the time at which a consi- 

 derable number of parts in the foetus are formed, and the heart 

 and bloodvessels have appeared. More lately, Baer, Prevost, 

 and Dumas, from their intimate acquaintance with the pheno- 

 mena and laws of development, have been more successful in 

 their researches on this subject. 



The older authors were aware that the ova found in the ovi- 

 ducts and uterus were derived from the Graafian vesicle of the 



