270 Dr Allen Thomson an the Vascular System 



IV. The general resemblance which the changes of de- 

 velopment in the ova or foetuses of vertebrated animals bear to 

 one another, is very striking ; it illustrates the analogy of struc- 

 ture in the different animals of this class when arrived at their 

 state of maturity, and seems to indicate very clearly, that* the 

 general plan upon which their systems and organs are construct- 

 ed and arranged is the same. This correspondence, indeed, in 

 the relations of organs to one another, and similarity in their 

 construction, to which the name of Type of Organization has 

 been given, appears to be comparatively much more clearly un- 

 derstood, from the knowledge, confined as it yet is, of the de- 

 velopment and transformation of the foetus, than from the 

 immense number of facts which have been established by the 

 examination of the structure of animals in their perfect state. 



In examining the embryoes of the vertebrated animals, at a 

 corresponding period in the early stages of their development, 

 it is truly surprising to remark how very much they resemble 

 one another. In some of the higher orders especially, it would 

 be difficult for those unaccustomed to such investigations, to 

 distinguish between the embryoes of the lizard, bird, or mammi- 

 ferous animal, when they are removed from their ova and divest- 

 ed of the accessory membranes ; and impossible for the expe- 

 rienced eye even, to perceive the differences between the embryoes 

 of the different families of the same order, as of birds or mam- 

 malia. 



The following may be regarded as some of the more important 

 particulars in which the phenomena of development correspond in 

 the different orders of vertebrated animals : — 1. The ovum being 

 essentially composed of a yolk, or collection of granular substance, 

 enclosed in a membrane, along with some accessory parts, all 

 enveloped by a general covering, to which the name of chorion 

 may be most properly applied. 2. The existence of some part 

 of the yolk, generally of a firmer consistence than the rest, 

 situated towards the upper surface, and of a membranous form, 

 called the germinal membrane, in which the changes connected 

 with the formation of the foetus more immediately take place. 

 3. The commencement of the formation of the embryo being 

 indicated by the appearance of a streak and small groove, in the 

 centre of the germinal membrane, and this groove afterwards 



