lxxxviii report — 1877. 



entiation of the two primitive layers of cells representing the persistent 

 ectoderm and endoderm *. 



If, pursuing this idea, we take a survey of the whole animal kingdom 

 in its long gradation of increasing complexity of form and structure from the 

 simplest animal up to man himself, we find that all the various modifications 

 of organic structure which present themselves are found, in the history of the 

 individual or ontological development of the different members of the series, 

 to spring originally from two cellular laminae, ectoderm and endoderm, the 

 component elements of which may again he traced back to the first segment- 

 sphere and primitive protoplasmic elements of the ovum. 



Time does not admit of my conducting you through the chain of observa- 

 tion and reasoning by which Haeckel seeks to convince us of the universal 

 applicability of his theory ; but I cannot avoid calling your attention to the 

 extremely interesting relation which has been shown to exist between the 

 primary phases of development of the ovum and the foundation of the blasto- 

 derm in very different groups of animals, more especially by the researches 

 of Haeckel himself, of Kowalevsky, Edward Yan Beneden, and others, and 

 ■which has received most efficient support from the investigations and writings 

 of E. Ray Lankester in our own country ; so that now we may indulge 

 the well-grounded expectation that, notwithstanding the many and great 

 difficulties which doubtless still present themselves in reconciling various 

 forms with the general principle of the theory, wo are at least in the track 

 which may lead to a consistent view of the relations subsisting between the 

 ontogenetic, or individual, and the phylogenetic, or race history of the for- 

 mation of animals and of man. 



In all animals, then, above the Protozoa, the ovum presents, in some form 

 or other, the bilamiuar structure of ectoderm and endoderm at a certain 

 stage of its development, this structure resulting from a process of segmen- 

 tation or cell-cleavage ; and there are three principal modes in which the 

 double condition of the layers is brought about. In one of these it is by 

 inward folding or invagination of a part of the single layer of cells immediately 

 resulting from the process of segmentation that the doubling of the layers is 

 produced ; in the second, perhaps resolvable into the first, it may be described 

 rather as a process of enclosure of one set of cells within another ; while in 

 the third the segmented cells, arranged as a single layer round a central 

 cavity of the ovum, divide themselves later into two layers. But the dis- 

 tinction of ectodermic and endodermic layers of cells is maintained, whether 

 it be primitive and manifested from a very early period, or acquired later by 

 a secondary process of differentiation. Thus in many Invertebrates, as also 



* At this place I will only refer to one of the most recent of Haeckel's works, in which 

 the views alluded to above are fully exposed in a series of most interesting memoirs, viz. 

 ' Studien zur Gastrasa-Theorie,' Jena, 1877 ; and to Dr. E. Percival Wright's translation 

 of the account of Haeekel'i views in Joum. of Microsc. Science, vol. xiv. 1874. 



