ADDK1SSS. XC1 



It is interesting to find that there is a correspondence between the 

 later division of the mesoderm of the higher animals derived from the two 

 primitive blastodermic laminae and the original absence of mesodermic 

 structure in the lowest animals, followed by tho gradual appearance, 

 first of one layer (the external muscular in the higher Coelenterata), and 

 soon afterwards by the two divisions or laminae with the intermediate 

 ccelom. 



In this account of what may be termed the organized foundation of the 

 new being, I have entered into some detail, because I felt that our conception 

 of any relation subsisting between the ontogenetic history of animals and 

 their phylogenetic evolution can only be formed from the careful study 

 of the earliest phenomena of embryonic organization. Notwithstanding the 

 many difficulties which unquestionably still block the way, I am inclined 

 to think that thero is great probability in the view of a common bilaminar 

 origin for the embryo of all animals above the Protozoa, and that the 

 Vertebrate equally with the Invertebrate animals may be shown to possess, 

 in the first stages of their blastodermic or embryonic formation, tho two 

 primitive layers of ectoderm and endoderm*. 



To attempt, however, to pursue the history of the development of animals 

 in detail would be equivalent to inflicting upon you a complete system of 

 human and comparative anatomy. But I cannot leave the subject abruptly 

 without an endeavour to point out in the briefest possible manner the bearing 

 of some of the leading facts in embryology upon the general relation of onto- 

 geny and phylogeny. 



We are here brought into the contemplation of those remarkable changes, 

 all capable of being observed and demonstrated, by which the complex 

 organization of the body of man and animals is gradually built up out of the 

 elementary materials furnished by the blastodermic layers— a process which 

 has been looked upon by all thoso who have engaged in its study with the 

 greatest interest and admiration. By comparing these phenomena as observed 

 in individuals belonging to different classes and orders of animals, it is 

 found that not only are they not different, but, on the contrary, that 

 they present features of the most remarkable resemblance and conformity, 

 and we are led to the conclusion that there is a general plan of development 

 proved to extend to the members of considerable groups, and possibly capable 



* If we reserve the words ectoderm and endoderm to designate the two layers of the 

 primary bilaminar blastoderm, we may apply the terms epiblast and hypoblast to their 

 derivatives after the formation of the mesoderm, and indicate the relations of the whole 

 to the secondary or quadrilaminar blastoderm by the following Table : — 



r Ectoderm ... f Epibkst } 



Primarv ^r ^ f Somatopleuro I Secondary 



BkstodZi Mesoderm . . . | Splancmiopleure .. bias 



(.Endoderm ... { Hypoblast 



