Address delivered at the Plymouth meeting of the British Association for the Advancement of Science, August 15, 1877 / by Allen Thomson.

Allen Thomson

Date:: 1877

Credit: Address delivered at the Plymouth meeting of the British Association for the Advancement of Science, August 15, 1877 / by Allen Thomson. Source: Wellcome Collection.

Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.

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$It ia 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 the gradual appearance, first of one layer (the external muscular in the higher Ccelenterata), 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 there 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 he shown to possess, in the first stages of their blastodermic or embryonic formation, the two primitive layers of ectoderm and endoderm*. To attempt, however, to pursue the history of the development of animals in detail would he 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 those who have engaged in its study with the greatest interest and admiration. By comparing these phenomena as observed in individuals belonging to diff’erent 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 quadrilaininar blastoderm by the following Table :— ' Epiblast j Ectoderm Primary Blastoderm Endodei iu ... | Mesoderm r Somatopleure i Secondary \ Splanchnopleure .. [Blastoderm. Hypoblast .. ■$