Primitive Cell-layers of the Embryo. 323 



the study of geographical distribution, furnishes, with regard to 

 such groups of organisms as have been preserved in the con- 

 dition of fossils, a distinct and independent mass of evidence, 

 enabHng the naturalist to sketch out parts of the genealogical 

 tree, thus supplementing and independently reiterating the 

 conclusions drawn from embryology. 



It is only within the last ten, or, we may almost say, the 

 last five years that the development of animals, especially of 

 the Invertebrata, has begun to be studied with the requisite 

 minuteness. Stimulated by the Darwinian theory and the 

 recapitulation hypothesis, naturalists are beginning to apply 

 the highest powers and new methods * of examination to the 

 study of the development of all kinds of organisms, so as to 

 trace out cell by cell the complete history of the elaboration 

 of the complex adult from the simple ovum. It is only now 

 that the first changes in the &^g (the first dispositions of the 

 embryonic cells) are becoming known in a sufficiently widely 

 varied series of form's to enable the naturalist to form genera- 

 lizations. It is only by slow degrees that those species are 

 being found out which conserve precious records in their 

 pregnant infancies, often not even hinted by the uneventful life- 

 histories of their nearest congeners. A commencement only has 

 been made, but one of great promise, by the researches of 

 Fritz Miiller ('Fiir Darwin'), Weissmanf, KowalewskyJ, Ed. 

 van Beneden§, Hackelll, and others, from which we may, I 

 think, draw conclusions of the greatest importance for genea- 

 logical classification. 



It would not be surprising if the facts of development were 

 to lead to another primary grouping of the animal kingdom 

 than that indicated in the four Cuvierian types or the six or 

 seven types now generally adopted, or should assign to 

 those great divisions unequal significance. They are con- 

 fessedly groupings based upon the anatomy of the adult or- 



* The method of hardening the developing egg, imbedding it in a 

 matrix, and then cutting thin sections, has only quite recently been ap- 

 plied to Invertebrata, chiefly by Russian naturalists. 



t Embryology of the Diptera (Zeitschr. fiir wiss. Zool. 1865-06). 



\ A series of papers, in the Memoirs of the Imperial Academy of St. 

 Petersburg (1867-71), on the development of Ctenophora, A.8C\A.\&, Amphi- 

 oxtfs, Sagitta, Euaxes, Lumhricus, Apis, and Ilydrophilus. 



§ A series of papers on the development of the (irefiarina of the lobster 

 and of various Crustacea {Nehalia, 31i/sis, Sacculina, &c. ), in the Bulletins 

 of the Belgian Academy, 1869-72. Also prize memoir, in the same 

 Academy's Transactions,' on "The Signification of the Parts of the Egg." 



II Moiiograph of the Monera, Jenaische Zeitschr. 1868 ; Generelle Mor- 

 phologic, 1867 ; The Organization of the Sponges and their Relationship 

 to the Corals, Jenaische Zeitschr., and Ann. k Mag. Nat. Hist. 1870, v. 

 pp. 1 \- 107. 



21* 



