LECTURES ON EMBRYOLOGY. 



tres of concentrated mass will begin to separate, 

 as we have here (Plate XXXVII, fig. 2) where we 

 have two distinct spheres, with a concentrated mass 

 in each. At this period, each of these concentrated 

 masses is without an envelope. And now there 

 will be an envelope formed around it. And here 

 it will grow into a hollow vesicle ; and as soon as 

 this last process has taken place, we have a free 

 egg. Around the spheres of condensed gran- 

 ules a membrane is formed, and some one or sev- 

 eral of the granules within growing larger, give 

 rise to a perfect egg. And so we see in the larger 

 and still larger, those concentrated collections take 

 place and go on developing as we have them here 

 (Plate XXXVIII, fig. B.) with a mass of con- 

 densed yolk,swimming in a granulated liquid. And 

 then the eggs escape from these pouches, and are 

 laid, under their normal form. Then begins the 

 series of modifications and repeated divisions 

 and subdivisions which give rise to the formation of 

 a germ to form a new individual. Now, the 

 changes of these eggs illustrate the same time 

 the formation of cells. They are multiplied by the 

 division of vesicles containing a simple liquid. 

 Condensation takes place within and around this 

 collection of granules, and a membrane is produced. 

 Then will appear again some granules growing 

 within, which will be the nucleoli. 



It can now no longer be doubted, that the pro- 

 cess of formation of eggs and the process of 

 formation of cells, are identical, as it was under- 

 stood that eggs and cells, in their perfect formation, 

 were similar organizations. 



I would now proceed to illustrate the further 

 changes of the germ of Mollusks to show how the 

 young of the Mollusks are developed how they 

 successively assume the form of the perfect ani- 

 mal, and how their various organs are developed. 

 Here is a diagram (Piate XXXIX), which gives a 

 general view of the rapid successive changes which 

 the eggs of Cuttle Fishes undergo, in which the 

 germ is formed around the yolk (Fig. B). After 

 some changes, the outline of the young animal is 

 formed (Fig. E), and after some other changes 

 (Fig. F), it begins to resemble the ful! grown ani- 

 mal (Fig. G) ; and before the animal is hatched, we 

 see it really does resemble the Cuttle Fish. (Plate 

 XXXVI, fig. A). 



You see ( Plate XXXIX, fig. G.) the body,the eyes 

 the tentacles, &c. But in order to show that all 

 Mollusks have the same mode of formation, not- 

 withstanding their apparent diversity, I must be- 

 gin by showing you that the perfect animals them- 

 selves are constructed upon the same plan. And 

 this is no easy task. There is no group of the ani- 

 mal kingdom which has been more studied, and no 

 one which is less understood than that of the Mol- 

 lusca in their morphology. I do not say that 

 there is no group in which species are less known. 

 On the contrary, few departments of the animal 

 kingdom have been more extensively studied in 

 the details in the distinction of genera and species. 



[PLATE XXXIX EMBRYOS OF THE CUTTLE 



FI-H I 



ot them have heeu vvt-H ti_;urcu C4ii-i de- 

 scribed. But the correspondence of their parts, 

 from one class to another the analogv of the dif- 

 ferent organs in their various positions this is 

 what is not understood in this class of animals. 



That all Mollusks agree in the softness of their 

 bodies, is well known. And this character has 

 .been constantly insisted upon as the distinguish- 

 ing character of Mollusca a soft, contractile body 

 without articulation. This is the general charac- 

 ter assigned to the type of Mollusca. And in ad- 

 dition to their character, derived from the external 

 appearance, is usually added the fact that they 

 have a nervous system, consisting of a circular 

 ring around the entrance of the alimentary canal, 

 with a swelling above and below, forming a single 

 ring without a chain of repeated swellings, as is 

 observed among the Articulata. But that Mollusks 

 agree beyond this, in their structure, is so little 

 understood, that in our descriptions,we find groups 

 contrasted in which it is said that the gills are 



