986 OP GENERATION. 



that stage of completeness in which it is able to bear a part in the vital economy 

 of the new being and embracing, also, the succession of changes in the pro- 

 visions for the nutrition of the embryo in the successive phases of its existence, 

 and the adaptations of its general organization to each respectively constitutes 

 one of the most interesting departments of Physiological Science, and one 

 which has of late years received more attention than any other. It is a branch 

 of the inquiry, however, which has, and seems likely to have less practical 

 bearing than any other ; for neither as regards fhe preservation of the body in 

 health, nor in its restoration from disease, is it easy to see what direct benefit 

 the most exact knowledge of Embryonic Development is likely to afford. The 

 chief subject on which it throws light, is that of Congenital Malformations and 

 Deficiencies ; many of which are now distinctly traceable to arrest or irregu- 

 larity of the developmental processes, some of them, indeed, to excess ( 596). 

 For these reasons, the topic before us will be passed over much more lightly 

 in the present Treatise than its scientific importance might seem to demand ; 

 and all that will be here attempted, will be a mere sketch of the mode in which 

 the evolution of the germ takes place, this being followed in the first instance 

 as a whole, whilst its principal organs will be afterwards separately considered 

 as they successively present themselves. 



993. This sketch, however, will serve to convey an idea of the nature of the 

 process, and to illustrate its conformity in Man to that great law of progress 

 from the general to the special, which is equally manifested in the development 

 of every other organized being. For, when we first discern the primordial cell 

 which is to evolve itself into the Human organism, we can trace nothing that 

 essentially distinguishes it from that which might give origin to any other form 

 of organic structure, either Vegetable or Animal ; its condition, in fact, being 

 permanently represented by the humblest single celled Plants and Animals. 

 The earliest stage of its development consists in simple multiplication by dupli- 

 cative subdivision, so that a mass of cells comes to be produced, amidst the 

 several individuals of which no difference can be traced ; and this also finds its 

 parallel among the simpler organisms of both kingdoms. Soon, however, this 

 homogeneous condition gives rise to a heterogeneous one ; the further changes 

 which different parts of the mass undergo not being of the same uniform cha- 

 racter, so that a marking out of organs, or instrumental parts adapted for differ- 

 ent purposes in the economy, comes to be discernible. The organs, however, 

 whose distinctness first becomes apparent, are not usually those which we trace 

 in the completed structure, but have a merely temporary character ; being 

 evolved either as a sort of scaffolding or framework for the building up of the 

 more permanent parts, or with a view to the nutrition of the embryo during the 

 evolution of these. Although the first indications of heterogeneousness in the 

 germinal mass are of nearly the same kind in all animals consisting in the 

 formation of a Hastodermic membrane (composed, however, of nothing else than 

 layers of cells) upon its exterior, which serves as a sort of temporary stomach, 

 whilst a large part of the included mass undergoes liquefaction, and serves as 

 the nutrient material for the tissues which are to be evolved from it yet indi- 

 cations are very speedily manifested, of the primary division of the Animal 

 Kingdom, of which the new being is a member j thus, in the case of the Hu- 

 man embryo, as that of all Vertebrated animals, the first indication of the per- 

 manent organization is shown in the " primitive trace" which marks out the 

 line of the vertebral column (Plate II. Fig. 11) ; and in this we very soon dis- 

 cern the foundations of the separate vertebra (Fig. 12, c). But there is no- 

 thing at this period to distinguish the germ of Man from that of any other Ver- 

 tebrated animal, this early part of the developmental process being carried on 

 upon the same plan in every member of that sub-kingdom ; and it is not until 

 we meet with indications of the plans which are peculiar to the respective 



