404 TRANSACTIONS OP SECTION D. 



their pliylogenetic history, are two undertakings whose ways for the present lie 

 in different directions. The more difficult task falls to the lot of the physio- 

 logical study of form. But if the pursuit of this study demands a concen- 

 tration of energy and a renunciation of the pleasure of frequent indulgence" in 

 wide generalisations, nevertheless it affords the priceless compensation of close 

 contact with the basis of our knowledge of Nature, and to him who follows 

 it with care and perseverance will be granted that sharpness of insight and 

 confidence of judgment which are the characteristics and reward of every exact 

 method.' 



His laid down two laws as the basal principles of the new science. The 

 first is the principle of ' Specific Organ-forming Regions of the Geem,' the 

 second is the 'Principle of DirrEEENTlAL Growth.' The first principle 

 affirms that the apparently undifferentiated germ is divided into different 

 regions in each of which are situated the materials for the. formation 

 of a definite primary organ. It follows that development really consists in the 

 formation of a mosaic of rudiments, each gifted with its own specific rate of 

 growth. The second principle affirms that the rate of growth of these various 

 rudiments are unequal, and that in consequence of the lateral pressures thus 

 set up various types of folding invagination, and other forms of embryogenetic 

 process must result : thus, for instance, His endeavours to explain the separation 

 of the myotomes from the lateral plate in the chick-embryo by the arching up 

 of the dorsal surface which takes place between the second and third days 

 of incubation. Since, according to His, the myotome is attached to the skin, 

 it is pulled upwards along with this and torn away from the lateral plate, 

 "which remains below, as this is tied to the yolk. 



Indeed, this book, which may be termed the first primer of Experimental 

 Embryology, is largely occupied by showing how secondary displacements of 

 embryonic organs must result from inequalities of growth : its great defect is 

 the absence of experiments to prove that these secondary changes reaUy are 

 the consequences of the primary changes to which His referred them. 



To Roux belongs the credit of being the first to make the decisive appeal to 

 experiment. In 1888 he published an account of how he had been able to 

 produce half-embryos of the frog by stabbing and killing one of the first two 

 blastomeres of the developing egg with a red-hot needle. In this way he 

 obtained half-blastulae and half-gastrulse, and even older half-embryos, with half 

 a nerve cord and half a notochord. Later he extended his experiments to 

 destroying the anterior two cells or the posterior two cells of the four-cell 

 stage, and claimed in this way to have obtained anterior and posterior half- 

 embryos. 



These experiments seemed to supply a solid basis of fact for the first 

 principle of His, viz. that of specific organ-forming areas in the germ ; but a 

 most unexpected further discovery by Roux was that of the phenomenon which 

 he termed ' Post-generation.' These half -embryos carried about attached to 

 them the dead blastomere (or blastomeres) which had been destroyed by the 

 experiment. This mass occupied, of course, the position which should have 

 been taken by the missing half of the embryo if the embryo had been a perfect 

 one. Now the half-embryos occasionally survived, and when this occurred the 

 missing half was regenerated, or, as Roux phrased it. Post-generated. Accord- 

 ing to Roux this took place by the migration of nuclei from the living into the 

 dead half by which the latter was recalled to life, and began to divide into 

 cells which then became moulded into the missing half of the embryo. 



Roux's position was strongly attacked by Hertwig, who maintained that 

 Roux had not succeeded in producing any real half-embryos, but that when 

 one blastomere had been killed the other began to develop into a whole 

 embryo; that the processes of folding, invagination, &c., which normally lead 

 to this result were impeded by the presence of the mass of dead yolk, and thus 

 a distorted embryo was produced which Roux had mistaken for a half one. 

 Hertwig pours ridicule on Roux's idea that nuclei could migrate into and 

 revivify a mass of protoplasm killed by being scorched by a red-hot needle, 

 and in subsequent publications Roux receded to the position that the post- 

 generation was due to the production of new cells by the uninjured half of 

 the egg, and that the dead half was only used as food ; but he steadfastly 



