922 OF GENERATION EMBRYONIC DEVELOPMENT. 



aggregation of simple cells formed by the duplicative subdivision of the 

 primordial vesicle, to 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 suc- 

 cession of changes in the provisions for the nutrition of the embryo in the 

 successive phases of its existence, and the adaptations of its general organ- 

 ization to each respectively constitutes one of the most interesting depart- 

 ments of Physiological Science, and one which has of late years received a 

 peculiar degree of attention. 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 the preservation of the body in health, nor its restoration 

 from disease, is it easy to see what direct benefit the most exact knowl- 

 edge 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 irregularity of the 

 developmental processes; some of them, indeed, to excess ( 357). 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 con- 

 sidered as they successively present themselves. 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. 



773. When we first discern the primordial cell which is to evolve itself 

 into the Animal organism, we can trace nothing that essentially distinguishes 

 it from that which might give origin to any other form of organic structure; 

 its condition, in fact, being alike in all, and permanently represented by the 

 humblest single-celled Plants and Animals. The earliest stage of its de- 

 velopment consists in simple multiplication by "duplicative subdivision" of 

 its contents, so that a mass of cell-like bodies comes to be produced, amidst 

 the several components 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 this mass undergo, not being of the 

 same uniform character, so that a marking out of organs, or instrumental 

 parts adapted for different purposes in the economy, comes to be discernible. 

 A marked divergence occurs, however, at a very early period, according to 

 whether the whole contents of the ovum undergo segmentation, and arc- 

 directly converted into the growing embryo, or whether only a portion is thus 

 segmented, the remainder forming a bag over which vessels are developed, 

 effecting the absorption of its contents, and thus enabling it indirectly to 

 participate in the process of development. Animals in which the former 

 arrangement holds are termed holoblastic, and are represented by Mammals 

 generally, Batrachia and Cyclostomata amongst the Vertebrata, and by the 

 simpler forms of Crustacea and Arachnida, the Annelida, lower Mollusca, 

 Entozoa, and Radiata amongst the Invertebrata. The animals in which the 

 latter arrangement holds are termed rneroblastic, and are represented by the 

 Mouotremata, Birds, scaly Amphibia, Plagiostomous and Teleostean iishos 

 amongst the Vertebrata, and by the higher orders of Crustacea and Arach- 

 nida, and by Cephalopoda amongst the Invertebrata. In the fowl, accord- 

 ing to His (loc. cit., p. 39), the whole of the nervous system, the whole of 

 the muscular system (both striated and unstriated), the true epithelial struc- 

 tures, and the glands proceed from the development of the morphological 



