4-SS 



THE POPULAR SCIENCE MONTHLY.— SUPPLEMENT. 



bryology, only two can be mentioned, viz., Caspar 

 Frederick Wolff, of St. Petersburg, well known as 

 the author of a work entitled " Theoria Genera- 

 tionis," published in 1759, by which the epic/enesis, 

 or actual formation of the organs in a new being, 

 was first demonstrated ; and Christian Pander, 

 who by his researches made at Wiirzburg ex- 

 plained, in a work published in 1817, the princi- 

 pal changes by which the embryo arises and is 

 formed. 



Von Baer was born in the Russian province 

 of Esthonia, on February 29, 1792. After having 

 been fifteen years professor in the Prussian Uni- 

 versity of Konigsberg, he was called to St. Pe- 

 tersburg, and having some years later been ap- 

 pointed to a newly-established professorship of 

 Comparative Anatomy and Physiology, he re- 

 mained in that city for nearly thirty years as the 

 most zealous and able promotor of scientific edu- 

 cation and research, stimulating and guiding all 

 around him by his unexampled activity, compre- 

 hensive and original views, sound judgment, and 

 cordial cooperation. In 1868, at the age of seven- 

 ty-six, he retired to T/orpat, from the university 

 of which he had received his degree in 1814, and 

 continued still to occupy himself with working 

 and writing in his favorite subjects, as well as 

 interesting himself in everything that was related 

 to educational and scientific progress, to very 

 near the time of his death, which occurred on 

 November 28, 1876, in his eighty-fifth year. 



Although Von Baer's researches, according to 

 the light in which we may now view them, con- 

 tributed in no small degree to the introduction of 

 the newer views of the morphological relations 

 of organic structure which have culminated in 

 the theory of descent, yet he was unwilling to 

 adopt the views of Darwin ; and one of his latest 

 writings, completed in the last year of his life, 

 was in vigorous opposition to that doctrine. 



It would have been most interesting and in- 

 structive to trace the history of the progress of 

 discovery in embryology from the period of Von 

 Baer down to the present time, but such a his- 

 tory would not be suitable to the purpose of this 

 address ; and I can only venture here, in addition 

 to Rathke, the colleague of Baer in Konigsberg, 

 to select two names out of the long list of distin- 

 guished workers in this field during the last forty 

 years, viz. : Thomas Bischoff, of Giessen and 

 Munich, to whom we owe the greatest progress in 

 the knowledge of the development of mammals, 

 by his several memoirs, appearing from 1842 to 

 1854; and Robert Remak, of Berlin, whose re- 

 searches on the development of birds and batra- 



chia, appearing from 1850 to 1855, gave greatly- 

 increased exactness and extension to the general 

 study of development. 



The germinal element, from which, when fer- 

 tilized, the new animal is derived, is contained 

 within the animal ovum or egg — a compact and 

 definite mass of organic matter, in which, not- 

 withstanding great apparent variations, there is 

 maintained throughout all the members of the 

 animal kingdom, excepting the protozoa, which 

 are destitute of true ova, a greater uniformity in 

 some respects than belongs to the germinal prod- 

 uct of plants. 



Usually more or less spherical in form, the 

 animal ovum presents the essential characters of 

 a " complete cell," in the signification given by 

 Schwann to that term. The germinal substance 

 is inclosed by an external vesicular membrane or 

 cell-wall. Within this covering the cill-substanee, 

 generally named yolk or vitellus, from the analo- 

 gy of the fowl's egg, consists, to a greater or less 

 extent, of a mass of protoplasm, and imbedded in 

 this mass, in a determinate situation, there is 

 found a smaller internal vesicular body, the ger- 

 minal vesicle or nucleus, with its more or less con- 

 stant or variable macula or nucleolus. 



Now, the first thing which strikes us as re- 

 markable connected with the ovum is the very 

 great variation in size as compared with the en- 

 tire animal, while in all of them the same simple 

 or elementary structure is maintained. The ovum 

 of mammals is, for example, a comparatively small 

 body, of which the average diameter is about the 

 T .l,77 of an inch, and which consequently scarce- 

 ly weighs more than- a very minute fraction of a 

 grain, which may be calculated perhaps only at 

 the TJuiTo part. And further, in two animals 

 differing so widely in size as the elephant and the 

 mouse, the weights of which may be held to 

 stand toward each other in the proportion of 

 150,000 to one, there is scarcely any difference in 

 the size of the mature ovum. 



On the other hand, if we compare this small 

 ovum of the mammal with the yolk of the egg in 

 the common fowl, the part to which it most nearly 

 corresponds, it may be estimated that the lat- 

 ter body would contain above 3,000,000 of the 

 smaller ova of a mammal. 



The attribute of size, however, in natural ob- 

 jects ceases to excite feelings of wonder or sur- 

 prise as our knowledge of them increases, whether 

 that be by familiar observation or by more scien- 

 tific research. We need not, at all events, on 

 account of the apparent minuteness of the ovum 

 of the mammifer, or of any other animal, have 



