298 Dr Allen Thomson on the Vascular System 



rudimentary parts of the foetus appear in vertebrated animals, 

 is composed of a thin layer of granular substance situated on the 

 surface of the yolk. This germinal membrane, or Blastoderms, 

 as it has been called in the egg of the bird, is the first part 

 which is seen to undergo a change : it becomes more distinct, 

 and gradually separates itself into three layers, called by Pan- 

 der the Serous, Mucous, and Vascular Layers of the Blas- 

 toderma, which, by the vai-ious folds they afterwards form, 

 give rise to the Nervous and Tegumentary, the Vascular and 

 the Intestinal Systems of the body. The layers into which 

 the germinal membrane becomes divided, are placed closely un- 

 der one another on the surface of the yolk. The outermost, 

 called the Serous Layer, is situated immediately under the mem- 

 brane of the yolk, and, by the changes which it undergoes in 

 the progress of evolution, forms the rudimentary state of the 

 brain and spinal cord, the parietes of the larger cavities of the 

 foetus, its muscular and osseous parts, its general integuments, 

 and proper envelope or amnios. The layer next in order has 

 been called Vascular, because in it the development of the prin- 

 cipal parts of the vascular system appears to take place. The 

 thii'd, called the Mucous layer, situated next the substance of 

 the yolk, is generally in intimate connexion with the vascular 

 layer ; and it is to the changes which these combined layers un- 

 dergo, that the intestinal, the respiratory, and probably also the 

 glandular systems owe their origin. 



The knowledge of these rudimentary parts of the germinal 

 membrane has, it is true, been obtained chiefly from the obser- 

 vation of the eggs of birds ; but later investigations render it 

 highly probable that they are also common to the ova of other 

 orders of vertebrated animals, and that they form a very strik- 

 ing analogy among these animals from the earliest periods of 

 their existence. 



In the following pages we purpose to describe some of the 

 changes which the middle, or vascular layer, undergoes in the 

 ova of vertebrated animals during the formation and develop- 

 ment of the more important organs which take their rise from 

 it ; but it will be necessary, first, to give a short account of the 

 general phenomena of development of the germinal membrane, 

 from the first signs of change, up to the period when the heart, 

 or principal circulating organ, is formed. As the development 



