PROCESS FROM THE GENERAL TO THE SPECIAL. 923 



elements of the cicatricula, whilst the blood and the connective tissues pro- 

 ceed from the so-called white yolk. The organs whose distinctness first 

 becomes apparent, are not (for the most part) 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 general mass are of nearly the same kind in all animals consisting in 

 the formation of a blastodermic membrane (composed, however, of nothing 

 else than layers of cells) upon its exterior, which serves as a sort of tempo- 

 rary stomach, whilst a large part of the included mass undergoes liquefac- 

 tion, and serves as the nutrient material for the tissues which are to be 

 evolved from it yet indications are very speedily manifested of the primary 

 division of the Animal kingdom of which the new being is a member; thus, 

 in the case of the Human embryo, as of that of all Vertebrated animals, the 

 first outline of the permanent 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 discern the foundations of the separate verte- 

 bne (Fig. 12, c). But there is nothing at this period to distinguish the germ 

 of Man from that of any other Vertebrated animal, this early part of the 

 developmental process being carried on upon the same plan in every member 

 of that subkingdom ; and it is not until we meet with indications of one of 

 the plans which are peculiar to the respective classes of that subkingdom, 

 that we can discover whether the germ in course of evolution is to become 

 a Mammal, Bird, Reptile, or Fish. So, even when it has been recognized as 

 belonging to the Mammalian class, there is at first nothing to distinguish it 

 from that of any other Mammal ; and it is only with the advance of the 

 developmental process that indications successively present themselves, which 

 enable us to distinguish, one after another, the characters of the order, the 

 family, the genus, the species, the variety, the sex, and the individual the 

 more special features progressively evolving themselves out of the more general, 

 which is the expression of the law of development common to all Organized 

 beings. 



774. With this progressive alteration in the condition of the embryo itself, 

 a very remarkable series of alterations is proceeding, paripassu, in the mode 

 in which it is supplied with nutrient material, and in the provisions for the 

 aeration of its circulating fluid. The first evolution of the germ takes place 

 entirely at the expense of the yolk: of which, however, the store contained 

 in the Mammalian ovum is very small. The whole of this is very speedily 

 incorporated in the substance of the germ, by the peculiar process to be 

 presently described ; and there is no residual store of " food-yolk," such as 

 that which, in the Bird, serves for the nutrition of the embryo during the 

 whole remainder of the developmental process, by being gradually absorbed 

 into the substance of the blastodermic membrane, and there converted into 

 blood. The Mammalian ovum, however, from the time it reaches the 

 Uterus, is furnished with a new supply of nourishment, in the fluid secreted 

 by the Decidual membrane ; and for the absorption of this, it is particularly 

 adapted by the villosities which develop themselves from its own external 

 envelope. These, at first entirely destitute of bloodvessels, are subsequently 

 penetrated at a certain part of the surface, by the foetal capillaries brought 

 to them by an organ, the Allantois, which is developed in Birds as the tem- 

 porary instrument of respiration ; and thus is originated the foetal portion of 

 the Placenta, of whose formation an account has already been given ( 753- 

 755). From the time that this organ is completed, up to the birth of the 

 Infant, the embryo draws its nutrient materials direct from the maternal 



