LECTURES ON EMBRYOLOGY. 



73 



between the embryogenic phenomena, as we have 

 just described them, and what is known of the ce- 

 lestial bodies, in their combinations, upon an im- 

 mense scale. First, we have primitive cells, com- 

 bining and condensing to form the mass of the 

 egg, like clusters of nebular stars. After the yolk 

 has undergone the various phases which precede 

 the formation of the embryo or germ, this new be- 

 ing with a spherical form, which is also the form 

 of the primitive egg, begins to assume a rotatory 

 movement, under the influence of life, as the ce- 

 lestial bodies rotate under the influence of univer- 

 sal gravitation. At last the progressive, onward 

 movement is introduced, which characterizes ani- 

 mal life properly, and is the first step in the series 

 of progress, which, in man, ends with intellectual 

 freedom and moral responsibility. 



But this form of the division of the yolk is not 

 the only one which is observed among animals. 

 In Fishes, for instance, we have a division of the 

 yolk, which differs considerably from that just de- 

 scribed. In these there will be first a transverse 

 depression upon the yolk, so that, seen from above, 

 the yolk will seem divided in two halves. And 

 then it will be divided again at right angles, so 

 that there will be two furrows at right angles, 

 forming a division which remains superficial. So 

 that in a profile view these furrows do affect the 

 yolk but very little, and the whole mass below re- 

 mains unaffected. 



But only the superficial layer undergoes this 

 change ; the lower portion and the central par^s of 

 the yolk remaining unchanged, but being gradu- 

 ally introduced into the process being gradually 

 absorbed by that part of the germ which is already 

 formed, and finally totally absorbed by the germ ; 

 or if not introduced into the substance of the germ 

 as a part of its body, it is finally introduced as a 

 sac from the lower part of the body into the di- 

 gestive cavity, and is digested. So that we have 

 all possible steps, from total division of the yolk, 

 which is entirely changed into a germ, to a super- 

 ficial furrowing giving rise to a germ which rests 

 upon a modified yolk. In the first instance, by 

 repeated subdivision, the whole substance of the 

 yolk is prepared to become a germ ; or, in the sec- 

 ond, only a part of it is modified to form a layer 

 upon the yolk, which grows and gradually absorbs 

 the remainder of the yolk. 



In those animals in which the division of the 

 yolk is only partial, as in fishes, the divisions 

 where they have been multiplied have nevertheless 

 finally given rise to cells. In the beginning, those 

 divisions are only separations of the superficial 

 mass. But those masses not being entirely sur- 

 rounded, do not form distinct spheres or parts of 

 spheres ; but at last, when they have repeatedly 

 multiplied, then each particle is surrounded by a 

 membrane, and thus transformed into a distinct 

 cell. So that the germ, in whatever manner it is 

 produced whether by total or partial division of 

 the yolk is finally, when formed, constituted of 



numerous small cells, The changes \vhich those 

 cells undergo the manner in which additional 

 cells are derived from the yolk, either by division 

 or by evolution from those already formed, con- 

 stitute the phases of the embryonic growth of each 

 animal. But it is by a uniform process of division 

 that the germ itself is first formed, The degree of 

 maturity which the germ has reached when it is 

 hatched, varies extraordinarily. There are ani- 

 mals in which the germ is hatched in a degree of 

 development which is so distant from what the 

 animal will be finally, that it cannot be recognized, 

 and that the type of the parent is not at all indi- 

 cated even in the outline, in the form, or in the 

 structure of the germ when born. There are other 

 animals in which, on the contrary, the germ is not 

 hatched before it has grown within the egg to 

 assume the external forms of the mature animal, 

 and has even attained to a very considerable size, 

 in many of them. 



It is perhaps from not having considered suffici- 

 ently those differences that so many mistakes have 

 been made in the study of the changes which those 

 animals undergo. Had it been supposed that ani- 

 mals were born in a condition in which they differ 

 so widely from the parent, they might have been 

 watched longer before they were described as dis- 

 tinct animals, on the sole ground that they were 

 free moving. And we should not find that animals 

 of the same species would be described under so 

 many different names if this had been more gene- 

 rally known. 



A great many larvae of Worms are undoubtedly 

 simply those small animals described as Infusoria ; 

 and I have myself seen eggs of Planaria give rise 

 to some of these Infusoria called Pararusecium, 

 Annellides, Here, for instance, is one (Flare XXVI, 

 figure E), remarkable for its sucker-like discs 

 [PLATE XXVI PARASITIC WORMS.] 



and the Cilia by which it moves. Tlie \oung 

 Planaria resembles closely *this species. And it 

 is more than probable it is altfiost certain that a 

 great number of those so-called Infusoria, are no- 

 thing more than the moving germs of Worms. 

 Here is, for instance, a young Planaria, in which 

 we have such a sucker, and in which the general 

 form reminds us of the Infusoria very striking- 

 ly. (Plate XXVII, fig. B). The change which 



