LECTURES ON EMBRYOLOGY. 



79 



vntrocKiced in one of cay preceding lec- 

 tures symbolical formulas for the three classes of 

 Radiate animals, 1 deem it usefEl to do the same 

 : for the Articslata. Thus the symbol of the whole 

 department will be an Omega (Plate XXXVI. fig. 

 A) representing th-e curious mode of formation of 

 the embryo at the inferior part of the vitellus, of 

 fcfee twe sides arise ia^r^er to -envelope the 



vitellus. For the class of Worms we will have the 

 same figure slightly opened at the summit, (Fig. B.) 

 IFiff.'C, an Omega with a transverse bar, will repre- 

 sent the class of Crustacea, where two regions are 

 already distract. Finally, Fig. D, with two trans- 

 verse bars, for the class of Insects, in which the 

 body is divided in three regions, 



LECTURE X 



tracing the nrst formation and the growth 

 'of animals, there is one point, which never should 

 "oe lost sight of. It is, that at various periods of 

 this growth, the substance of which the animal 

 consists gradually changes. 



We have seen that in the beginning the germ 

 consists of simple cells, derived from a modification 

 'of the yolk, Such is the first condition of all 

 germs. Now, from this starting-point we may ar= 

 Vive at animals so complicated as Man. 



In other animals, throughout the series of the 

 animal kingdom, in which the most complicated 

 structures are observed la which structures very 

 distinct are successively foreied, <fiesh, blood, 

 'nerves, skin, hairs, scales, and aR possible struc- 

 tures so different as scarcely to be compared 

 how are these formed *? Are they new things fa- 

 'troduced during the growth of the germ or are 

 'they only modifications, simple changes o'f one 

 and the same fundamental element, modifications 

 of the cellular tissue which characterized the germ 

 \vben forming 1 



This is a question which can be answered by 

 'facts which have been entirely investigated by one 

 gentleman, a young physiologist of 'Germany, Pro- 

 fessor Schwann. Ten years ago he began tc exam- 

 ine the subject of animal tissues, and up to that 

 time it was believed that animals and plants drSer- 

 ed widely, that their substance had nothing simi- 

 'lar, that cells existed only in plants. Such was 

 the condition of things in i&SS, when Sc&wann, 

 taking tip the beautiful investigations which 

 Schleitien ! !iad just published upon the structure 

 and growth of vegetable cell-s, came to the conclu- 

 sion that animal tissues consisted equally of cells, 

 and that whatever may be the cdtn plication of this 

 substance in the animal whatever may be the ex- 

 ternal form of the various parts in the animal tis- 

 sues they all originate from cells, and are, after 

 -all, only modified cells. 



?u tbis absolute form, .perhaps the -results l ctf 



Schwann will have to 'be sotnewnat modified, 'otft 

 in foe main all subsequent investigations have 

 only gone to confirm his unexpected result, and a't 

 present tnere is no student in Anatomy, who has 

 not seen these cells o'f animal tissues, who is not 

 able to nd them out, even with microscopes of a 

 very inferior quality. But it required the sagacity 

 of the able and persevering investigator whose 

 name I have mentioned, to start such an investi- 

 gationto go through with it to give Unfinished, tc 

 the world, and then to remain silent for ten years 

 through all the attacks he has "had to undergo. 



Since Schwann published the volume containing 

 the results of his investigations, he has net been 

 heard in the debates which are still going 'onnpon 

 this subject, It is a remarkable instance of con- 

 fidence in his theory, and of a desire not to inter- 

 fere with that which contradictory investiga- 

 tions might bring about. Still it is known by ; his 

 friends that he is pressing on, and preparing new 

 investigations, which may iea& to as important re- 

 sults as his preceding labors. 



His efforts now go to ascertain 'how these ceils 

 are combined to form individuals of different kinds. 

 Indeed, he has undertaken nothing less than to in- 

 vestigate, if possible, the principle which combines 

 those cells into individual cells, to ascertain the 

 nature of that power which we call vital power, to 

 find oat what kind of influence it is which consti" 

 tutes individual, independent 'and progressive be- 

 ings. 



i have delayed introducing this subject up to the 

 present evening, became there is no class in which 

 the cellular structure of animal tissues can be so 

 fully and easily illustrated, as among Mollusca. 

 In their tissue when full grown, in their egg when 

 forming, the celmlar structure is perfectly plain 

 and easily ascertained. 



To what important results for Physiology the 

 final investigations on this subject will lead, cas 

 scarcely be foretold now. 'For siace it has bees 



