ADDRESS. XCV11 



Iii its simplest and earliest form that plan comprises a minute mass t f 

 the common nitrogenous hydrocarbon compound to which the name of 

 protoplasm has been given, exhibiting the vital properties of assimilation, 

 reproduction, and irritability. The second stage in this plan is the nucleated 

 and enclosed condition of the protoplasmic mass in the organized cell. We 

 next recognize the differentiation of two productive elements, and their com- 

 bination for the formation of a more highly endowed organizing element in 

 the embryonic germ-sphere or cell ; and the fourth stage of advance in the 

 complexity of the organizing phenomena is in the multiplication of the fer- 

 tilized embryo-cell and its conversion into continuous organized strata, by 

 further histological changes in which the morphological foundations of the 

 future embryo or new being are laid. 



I need not now recur to the further series of complications in the formative 

 process by which the bilaminar blastoderm is developed and becomes trila- 

 minar or quadrilaminar, but only recall to your recollection that while these 

 several states of the primordial condition of the incipient animal pass insen- 

 sibly into each other, there is a pervading similarity in the nature of the his- 

 tological changes by which they are reached, and that in the production of the 

 endless variations of form assumed by the organs and systems of different 

 animals in the course of their development, the process of cell-production, 

 multiplication, and differentiation remains identical. The more obvious 

 morphological changes are of so similar a character throughout the whole, 

 and' so nearly allied in the different larger groups, that we cannot but 

 regard them as placed in some very close and intimate relation to the 

 inherent properties of the organic substance which is their seat, and the 

 over-present influence of the vital conditions in which alone these properties 

 manifest themselves. 



The formative or organizing property therefore resides in the living sul> 

 stance of every organized cell and in each of its component molecules, and is 

 a necessary part of the physical and chemical constitution of the organizing 

 elements in the conditions of life ; and it scarcely needs to be said that these 

 conditions may be as varied as the countless numbers of the molecules which 

 compose the smallest particles of their substance. But, setting aside all 

 speculation of a merely pangenetic kind, it appears to me that no one could 

 have engaged in the study of embryological development for any time without 

 becoming convinced that the phenomena which have been ascertained as to 

 the first origin and formation of textures and organs in any individual animal 

 arc of so uniform a character as to indicate forcibly a law of connexion 

 and continuity between them ; nor will his study of the phenomena of 

 development in different animals have gone far before he is equally strongly 

 convinced of the similarity of plan in the development of the larger groups, 

 and, to some extent, of the whole. I consider it impossible therefore for any 

 one to bo a faithful student of embryology, in the present state of science, 



