LECTURES ON EMBRYOLOGY. 



cuore particularly than before, upon the growth of 

 animals within the egg; and some facts have been 

 brought to light which have their bearing on Zoolo- 

 gy. Though how these facts have to be applied to 

 the study of classification, has not yet been traced. 

 Embryologieal investigations have been particular- 

 ly made with reference to Physiology that is, with 

 reference to the mode of formation of the various 

 orgatve which exist in animals, and not with refer- 

 ence to ascertaining their natural relation among 

 themselves* 



Another series of investigations which have mod- 

 ified considerably the views which were entertain- 

 ed of the structure of the animal kingdom, are 

 those microscopical researches upon the intimate 

 structure of the tissue of the mass of the body. 

 Of what does the flesh* the bone, the nerve, the 

 various masses of the body,consist ? and how have 

 they been gradually formed? has been the object 

 of various microscopical investigations. And 

 again, in this department facts have been brought 

 to light of whicii we can avail ourselves in inves- 

 tigating the natural relation of animals. On in- 

 troducing a series of Lectnres on Embryology, my 

 object is not to illustrate embryology in the same 



sense, in the same manner, in which it has gen- 

 erally been traced. 



I EGGS OF FISHES.] 



in the animal kingdom. My object is not merely 

 Embryology; it is Comparative Embryology. And 

 under Comparative Embryology, I mean the com- 

 parisons of those phenomena which have been 

 traced in the growth of the different animals, and 

 the different modifications which occur in individ- 

 dual species, throughout the different classes, in 

 their natural gradation, when full grown. 



Let me, with a reference to a few diagrams, 

 show what I mean. Here are the various stages of 

 the growth of a fish. See here [A] the egg in the 

 earliest condition. Here is the first indication of 

 something different (Bi Next we see it still fur 

 ther advanced. There are afterwards successive 

 changes taking place, which go on to give rise to an 

 elongated mass, J Plate I, C D E] which swells and 

 elongates more and more till in the anterior por- 

 tion there is a greater swelling, which finally as- 

 sumes a more decided change, till there are indi- 

 cations of longitudinal lines, which grow more 

 prominent. 



The transverse divisions are introduced until we 

 see a little fish is coming. [Laughter]. From this 

 time it undergoes another series of changes. IE 

 resembles more a fish. The head is now distinct. 

 The backbone appears here. It begins to be mov- 

 able and finally, [F] we have the form of the fish,. 

 with the mass of yolk under the abdom-en. 



Now, embryology traces all these changes from 

 the first formation of an egg to the formation of 

 the germ within the egg; but the germ is not yet 

 formed. We have next to witness the formation 

 of the animal; and afterwards we trace io the prim- 

 itive egg, the successive changes of the first rudi- 

 mentswe trace its transformations. We have 

 first its formation in the egg. We trace afterward 

 its transformation through changes of different 

 forms. And it is important to distinguish between 

 these two orders of phenomena the formation of 

 the germ, and the transformation of the animal 

 into different outlines. The one would be the sub- 

 ject of embryology proper; the other is called the 

 metamorphosis of an animal; and has been partic- 

 ularly studied among insects, where the new being 

 passes through very different and quite distinct 

 forms. For instance, in Butterflies it is first in the 



; form of a caterpillar, as you see here: [Plate II. 

 fig. A 3 



[PLATE IE BUTTERFLIES AND CATERPILLARS j 



I shall not undertake to so back to the begin- 

 ning of animal life, to attempt to illustrate in what 

 manner individual life is produced, and how, 

 generation after generation, new sets of individ- 

 uals of each kind are made to succeed each other. 

 I shall simply take the germs as they occur in the 

 egg, to trace the changes they undergo ; and by 

 the knowledge of such changes, show that they 

 orm such series as agree with the natural series j 



