98 



PROF. AGASS1Z S 



ods ; but compare these earlier forms (Plate IX.fi gs. 

 L to Q) with those of the Chicken, (Plate VII, figs. 

 E to I, ) and you see again that the outlines are iden- 

 tical. I might have had a figure of a young Rabbit, 

 when the head has come to be more distinctly de- 

 veloped, and when the posterior part is more con- 

 tracted, and the resemblance would be still greater 

 than you notice at present, on comparing those 

 lower figures, (Plate IX, fig. P, and Plate VII, fig. 

 H.) 



It is not only in these forms of the germ that we 

 have an identical structure, an identical form, an 

 identical process of the formation of the eggs, and 

 the same identical functions; we have the same 

 modifications in the egg to bring about the forma- 

 tion of a germ ; and this process by which the 

 germ is formed is identical in. Vertebrata with 

 what we have observed in Radiata, in Mollusca, in 

 Articulata. The resemblance does not go further, 

 but here the identity is complete. The egg of a 

 Fish (Plate I, fig. A) consists of a yolk mass, en- 

 closed in its membrane, and containing a germina- 

 tive vesicle and a germinative dot. If we pass on 

 to the classes which we would suppose to differ 

 most the Mammalia, for instance we find that 

 the egg of a Rabbit (Plate IX, fig. A) consists, as 

 you see, of a vitelline membrane, a germinative 

 vesicle, and within it a germinative dot. This is 

 the egg in its primitive formation, as it was dis- 

 covered by C. E. Von Baer, in the year 1827, when, 

 for the first time, the unity which exists in 

 the starting point of growth of all animals, was 

 clearly ascertained. You remember that we ob- 

 served an identical structure among other classes ; 

 and if I were to enter into the details of the 

 changes which the egg undergoes, I should seem- 

 ingly repeat only what I have said about other 

 types. If you have not forgotten what I said about 

 the divisions of the yolk, you will remember that 

 the egg showed (as represented in Plate I, fig. B,) 

 a successive division into cells, which is even more 

 regular in Mammalia than in any other class. It is 

 here ( Plate IX,figs.C to F) symmetrical in the Rab- 

 bit, with the division into halves, and the subdivi- 

 sion into quarters, into eighths, into sixteenths, 

 and into thirty-two parts, and so on, as represent- 

 ed in the various figures in this plate. And in- 

 deed, Professor Bischoff, who has traced all the de- 

 tails, represents the yolk as dividing first into two 

 masses, then into four, then into eight, sixteen, 

 thirty-two, and even sixty-four. Then the whole 

 vitellus is transformed into a uniform mass of 

 minute cells. Next a mass of somewhat different 

 substance is circumscribed. (Fig. G,) to be the 

 germ ; so that in describing the changes of sub- 

 stance occurring in Mammalia, we have the same 

 thing that we have noticed in the Worms, the Crus- 

 tacea, in the Shells, and which we know to take 

 place among Radiata. We know further, that these 

 changes (Fig. B) will go on to form organs, and 

 finally to produce the perfect animal. 



Such a germ is formed here in Birds, (Plate VII, 



fig. C). The mass in the middle of the yolk is a 

 part of its fatty contents. There is no funda- 

 mental difference, however, from what we observe 

 in the Mammalia; but as the substances are not 

 so completely mixed up as in other classes, the 

 figure, of course, represents the details as they ap- 

 pear. The young germ within the egg of the Fish, 

 (Plate I, fig. C), and also in the Frog (Plate III, fig. 

 A,) and the Snail, (Plate IF, fig. A,) represents also 

 the same condition. So that after the egg has 

 undergone its particular changes, the germ is 

 formed upon it, and has an identical aspect in the 

 four classes of Vertebrates. (PI. III. page S.) 



Now this germ will grow by a double process 

 of extension. It is thickened by the assimilation 

 or by the transformation of successively larger 

 portions of the yolk, which is introduced into the 

 mass by transformation of substance, and next it 

 expands over successively larger parts of the sur- 

 face of the yolk. Then the thickened substance 

 of the germ soon divides into several layers, as is 

 figured in the diagrams in Plate VII, fig. D, where 

 the germ having grown larger, we may distinguish 

 a circular outline, and another of a more elonga- 

 ted form ; and in a transverse section of such a 

 germ, (Plate VIII, fig. A), we may observe that 

 the upper portion of it has a somewhat different 

 size from that of the lower and middle layer. So 

 that in its thickness the germ begins to show it- 

 self by the changes which it successively under- 

 goes. As soon as this separation of the germ into 

 several layers has taken place, then each layer for 

 itself will undergo changes. The upper layer, es- 

 pecially, is thickened within the centre ; and in the 

 middle layer there is an accumulation taking place 

 about the centre also, and the lower layer is 

 growing very soon beyond the upper, and beyond 

 the middle layer. So that from the figure seen 

 from above, (Plate VII, fig. D),you see the out- 

 lines of the three layers at once. Successively the 

 layers will enlarge, so as to cover the greater por- 

 tion of the yolk. 



How the germ extends, gradually, more and 

 more, so as to cover a greater and greater portion 

 of the yolfc, is seen here (Plate IX, fig. J,) where 

 the layers are seen hanging over the yolk, and 

 finally, (Fig. K,) enclosing it in the lower cavity of 

 the germ ; or the upper parts cover only one por- 

 tion of the whole surface. To shorten this illus- 

 tration, let me at once say, that the upper layer is 

 the foundation for all those organs by which an- 

 imal life, properly, is maintained ; by which ani- 

 mal life, properly, is expressed. It is from this 

 layer that the whole frame which is acting in life, 

 will be developed. It is from this part that 

 the head, the legs, the walls of the body, the 

 flesh, the bones, the brain, the organs of sense, are 

 successively developed. The upper layer (Plate 

 VIII, fig- A,) gives, indeed, rise to the organs of 

 animal formation of all life ; and the middle layer, 

 on the contrary, gives rise to the formation of the 

 heart, and the blood vessels ; to the organs of the 



