PROF. AGASS1Z S 



so that the primitive mass of the yolk will be 

 divided into four equal parts. And then those 

 segments will be subdivided and subdivided, till 

 the whole mass consists of small yolklets, each 

 surrounded by a membrane. 



But the subdivision is accompanied by a pecu- 

 liar formation of other masses within those partial 

 spheres. Let me show you some diagrams repre- 

 senting this process. In Plate XXIV, fig. C, we 

 have the eggs of Planaria,ia which the yolk is divi- 

 ded into four masses ; and in Plate XXIV, fig.D, we 

 have it the same under slight pressure, when four 

 clear spheres are noticed within each of these seg- 

 ments. 



In the next place we observe that besides the 

 four great masses there are four small ones, rising 

 in the centre. 



Again we may observe in each of the small ones 

 such a clear sphere, and when the subdivision goes 

 on forming a greater number of these spheres, the 

 whole process is repeated, the large one being 

 greatly reduced, there being successively, 16, 32 

 or more. Such fragments are increased very reg- 

 ularly, and though many variations are observed, 

 they appear in multiples of two or four, and so on. 

 When it has gone on a certain time, instead of four 

 small ones and eight large ones, or vice versa, there 

 will be quite a number of minute ones, and all 

 alike in size, and the process will be repeated tiil 

 these divisions are so minute that it is no longer 

 possible to count them, they forming a mass of 

 little cells, filling the whole of the membrane of the 

 yolk. 



What those clear spheres within the yolk are, it 

 is somewhat difficult to say, inasmuch as chemi- 

 cal analysis cannot reach them. The eggs are so 

 small that their composition has not been exam- 

 ined. It is only with the microscope that we can 

 reach these processes and determine the changes 

 of form and substance which take place, by the 

 various properties of these substances with refer- 

 ence to light. 



The fact of their being more or less transparent 

 will make some appear different, under the mi- 

 croscope, from others. And that is the whole 

 ground upon which the changes can be ascer- 

 tained. 



The manner in which the division takes place 

 when there are two forming, for instance, in the 

 intestinal Worms, has been described by Dr. 

 Bagge, as follows. 



The primitive clear sphere in the centre is said 

 to assume an elongated form,, and then the centre 

 to be contracted, and finally the two ends become 

 independent by a separation of the middle part, so 

 as to form two spheres ; and then the yolk mass to 

 agglomerate around those transparent spheres; and 

 then a division to be formed in the vitelline mem- 

 brane; and that to go on and to divide the vitellus 

 into two spheres ; and in each the same process 

 having been repeated, to have transformed that 

 four. Assuming again an elongated form, and 



then dividing completely, they go on and foraa 

 four masses. But that clear spheres within do nofc 

 always constitute or determine the separation of 

 the substance of the yol& into more and more nu- 

 merous masses, is shown by the example which I 

 have quoted, where a clear space exists in thecerr 

 tre of an egg, and the division takes place across 

 it. For instance, there will be such a mass as rep- 

 resented ia Plate XXV, Sg. H., and the division 

 will take place, a clear sphere accumulating on 

 one side of the mass, aad she yolk condensing on 

 the other side, and so on. 



The fact is, that the subdivision of the yolk mass 

 and the formation of these clear spheres, is a pro- 

 cess which goes on .simultaneously, but which can- 

 not be considered as directly dependant on each 

 other. In proportion as this tendency of the yolk 

 to subdivide is manifested by a contraction of the 

 mass, and the division of the spheres into two- 

 spheres, in the same proportion the substance 

 within the yolk, which fills the space in the centre 

 of the yolk, accumulates in spheroid raasses, to* 

 give rise to partial spheres. And that beiag re- 

 peated, there are then numerous divisions of the 

 yolk successively introduced, and having been en- 

 tirely kneaded^ as it were, by this repeated divis- 

 ion, the substance of the yolk in process of time 

 becomes a germ. 



For instance, in the Worm from which the dia- 

 grams in Plate XXIV are made, the germ (Fig, 

 A,} is a mass of very minute cells. Then from 

 the surface of those cells rises vibratory Cilia. We 

 know that cells can have vibrating Cilia on one of 

 their extremities. It is observed in the full-^rown 

 animals, and it is observed ia many germs, es- 

 pecially in Mollusks, tbat such vibrating Cilia, 

 are formed on the external surface of cells and 

 become an apparatus for locomotion, which Cilia 

 are voluntary, ceasing to move at intervals, re- 

 newing their motion at other times and transport- 

 ing the animal from place to place. But remark- 

 able a& it is, tbat the sphere is the fundamental 

 form of all animals, so rotation is the form, of the 

 action of all animals whea they begin- to move 

 within the vifelline raembrane 



No sooner has the little Planaria (Plate XXIV) 

 been covered with vibrating Cilia, than it begins 

 to revolve upon itself ; it has then a spherical out- 

 line, and undergoes a rotatory, constant motion in 

 one direction to begin with. And whea it has 

 crown to assume a somewhat elongated form, by 

 which the prevailing longitudinal diameter will be 

 introduced, after that longitudinal diameter b&9 

 exceeded the transverse, then it will change the di- 

 rection. And as soon as it is hatched, then it wi!3 

 proceed in an onward and forward motion, which 

 will be the motion that will characterize the an- 

 imaH aad then comes tbe bilateral symmetry 

 which exists throughout the animal kingdom, even 

 where it is concealed under the radiated form of 

 so-called radiated animals. 



A remarkable comparison might b lmsfcitRtfl 



