ADDRESS. lxxXlX 



iu Ampliioxus among the Vertebrates, a distinct invagination occurs, •while 

 in Mammals, as recently shown by Yan Beneden's most interesting observa- 

 tions in the rabbit's ovum, and probably also iu some invertebrates, the cells 

 of the ectoderm gradually spread over those of the endoderm during the pro- 

 gress of segmentation, and thus the endodermic comes to be enclosed by the 

 ectodermic layer of cells. 



From tho very uovel and unexpected observations of Yan Benedcn it further 

 appears that from the earliest period in tho process of segmentation in the 

 mammal's ovum it is possible to perceive a distinction of two kinds of seg- 

 ment-spheres or cells, and that when this process is traced back to its first 

 stage it is found that the whole of the cells belonging to the ectoderm are the 

 progeny of, or result from the division of the upper of the two first formed 

 segments, and that the whole of the endodermic cells are the descendants of 

 the lower of the two first segmented cells. This, however, is not an isolated 

 fact belonging only to mammalian development, but one which very nearly 

 repeats a process ascertained to occur in a considerable number of the lower 

 animals, and it seems to promise the means of greatly advancing the compre- 

 hension of the whole process of blastodermic formation. Thus ectoderm and 

 endoderm, which are in fact the primordial rudiments of the future animal 

 and vegetative systems of the embryo, are traced back as distinct from each 

 other to the first stage of segmentation of the germ. 



Accepting these facts as ascertained, they may be regarded as of the deepest 

 significance in the phylogenetic history of animals ; for they appear to open 

 up the prospect of our being able to trace transitions between the earliest 

 embryonic forms occurring in the most different kinds of ova, as between the 

 discoid or meroblastic and the vesicular or holoblastic, through the inter- 

 mediate series which may be termed amphiblastic ova*. 



In the lowest animals, the two layers already mentioned, viz. ectoderm 

 and endoderm, are the only ones known to constitute the basis of develop- 

 mental organization ; but as we rise in the scale of animals we find a new 

 feature appearing in their structure, which is repeated also in the history of 

 the formation of the blastoderm in the higher animals up to man. This 

 consists in the formation of an intermediate layer or layers constituting the 

 mesoderm, with which, in by far the greater number, is connected the forma- 

 tion of some of the most important bodily structures, such as the osseous, 

 muscular, and vascular systems. 



I will not stop to discuss the very difficult question of the first origin of the 

 mesoderm, upon which embryologists are not yet entirely agreed, but will 



* I ought here to refer to the elaborate memoirs of Professor Semper on the morpho- 

 logical relatione of the Vertebrate and Invertebrate animals, contained in the ' Arbeiten 

 aus dem Zoolog.-zootom. Institut in Wurzburg,' 1875 and 1876, in which the conclusions 

 arrived at do not coincide with the views above stated. 



