LECTURES ON EMBRYOLOGY. 



97 



is as complete as it can be, though each of these 

 types grows to a complication of structure, by 

 which the young Mammal, for instance, leaving 

 behind this low organization of the lower types, 

 rises to a complicated structure, to higher and 

 higher degrees, and to that eminence even which 

 characterizes mankind. 



As it is out of the question for me to introduce 

 an illustration of all the phases of these changes, I 

 will only introduce such points of the subject, 

 as bear upon classification, and upon the succes- 

 sion of types in former geological ages, in or- 

 der to show that the principle which I intend to 

 introduce, as the fundamental principle of classifi 

 cation, is really of value, in all departments of 

 Zoology. 



In these diagrams you have representations of 

 the changes which animals of the four classes of 

 Vertebrata undergo. Here (Plate I, page 7) is the 

 history of a Fish, (a White Fish from Lake Xeuf- 

 chatel), as represented by Dr. Vogt, from the egg 

 (Fig. A) up to the period when the young Fish 

 (Fig. F) is hatched. The close resemblance be- 

 tween this form (seen in fie. H) and other classes, 

 is more striking. Here (Plate II, figs. F to 0) we 

 have the history of a Frog, (also from a paper of 

 Dr. Vogt,) from the first moment of its formation 

 (Fig. F.) up to the period when the young Tadpole 

 (Fig. N) is hatched. In Plate II, figs A to E, are 



[PLATE II EGGS OF SNAIL AND FROG.] 



the changes which a Snail undergoes, according 

 to the illustrations of lUthke; and in this fig- 

 ure (Fig. B,) it is represented, as it appears, 

 taken out of the egg, and deprived of its external 

 envelope, in order to compare it with the form of 

 the young Tadpole, (Plate II, fig. L,) or the form 

 of the young Fish (Plate I, fig. H.) You see the 

 Snail, in its early condition, resembles the young 



Tadpole, closely, as you may ascertain by compar- 

 ison of the figures. The resemblance with the 

 Fish is not less striking, as you see on comparing 

 also the figures of the young Fish. 



[PLATE VII. EGGS or BIRDS AND HENS.] 



And if we go on, we shall find the same agree- 

 ment in Birds and Mammalia. We have here (in 

 Plate VII) the Hen's egg. Here, (Figs.E to K) we 

 have the different changes within the egg, as fig- 

 ured by Pander and Baer. We have here the dif- 

 ferent modifications of the young Chicken within 

 the egg; and we have here (Fig. K) the young 

 Chicken, already formed, at one of its earlier peri- 

 ods of growth, when it has yet undergone slight 

 changes of form in its progressive development ; 

 and here, (Plate IX,) as we proceed further, we 

 have the history of the changes of the embryo f 

 a Rabbit, from the remarkable work of Prof. Bis- 

 choff. I have not figured the outlines of all peri- 



[PLATE IX EGGS or RABBITS.] 



