PRESIDENTIAL ADDRESS. 413 



regenerating organs receive an appropriate explanation. For instance, when the 

 tail of a lizard is broken off it not infrequently happens that two tails are 

 regenerated. This result can be artificially brought about by slightly injuring 

 the regenerating surface. Here, then, we have another illustration of the 

 principle that the number of organs of a given type produced by organ-forming 

 substance depends on the outline of the germ. Where this is in a uniform 

 curve, one is produced ; if it is indented, two are produced. 



Besides regeneration the phenomenon of budding is almost certainly to be 

 referred to renewed nuclear activity in the production of organ-forming sub- 

 stances. It has long been a puzzle why in so many cases the development of the 

 bud pursued a different course from that of the fertilised egg. Thus in the 

 development of the bud of Ascidians the central nervous system is developed 

 from the pharyngeal sac, whereas in the develoi^ment of the ovum it is formed, 

 as in the higher Vertebrates, from the ectoderm. But the ectoderm of the early 

 embryo, as Hjort points out, is a layer of cells consisting of undifferentiated 

 protoplasm, whereas the ectoderm of the bud is an extension of the maternal 

 adult ectoderm, a layer of cells of hopelessly specialised cells irrevocably com- 

 mitted to the production of cellulose for the formation of the test, -whose 

 character could not be changed by the injection of any amount of organ- 

 forming substance. Therefore the organ-forming substances are differently 

 distributed, and chiefly poured by the active nuclei into the cells of the more 

 plastic inner layer. If this view be admitted, we can see at once why the 

 capacity of reproduction by means of buds is in general limited to animals of 

 lowly organisation. It is not that the nuclei of the higher animals become 

 limited in their potentialities : it is that their cytoplasm becomes too specialised 

 to be modified in new directions. This is true even in the case of animals of 

 simple organisation if they possess a strongly specialised cytoplasm, as, for 

 instance, the Nematode worms. 



We have now taken a brief survey of the evidence for the existence of organ- 

 forming substances, elaborated by and emitted from the nucleus, which confer 

 on the cytoplasm the power of forming the primary organs of the embryo. 

 We have learnt that these substances aggregate themselves in centres, each of 

 which tends to form an organ, and we can easily see that any influence, external 

 or internal, which would tend to increase or diminish the number of the centres 

 would correspondingly increase or diminish the number of similar organs formed 

 from such substance. But, as we all know, these primary organs undergo 

 further differentiation during the course of development into the secondary and 

 definitive organs; and we shall now submit evidence that the formation of these 

 secondary organs is determined, not by substances emitted from the nuclei of 

 the primary organs to which they belong, but by substances absorbed from the 

 blood or body fluid which have been produced by other organs. The first 

 striking case of this was discovered by Herbst. As is well known, Crustacea are 

 able to regenerate their limbs if these be cut off. Now, Herbst found that 

 this is also true of the eye-stalk ; if this be removed from a young shrimp, it 

 will in time regenerate a new eye. But if the optic ganglion which lies 

 beneath the eye be likewise removed, then, when the wound heals up, there will 

 be produced, not a new eye, but an extra antenna. There seems to be no 

 escape from the conclusion that, in normal development, the influence which 

 compels the ectoderm to modify itself into the lenses, crystalline cones, and 

 rhabdomes of the compound eye must be emitted by the ganglion cells of the 

 optic ganglion. 



Another striking case has been brought forward by Lewis. In the develop- 

 ment of the Frog, as in that of other Vertebrata, the retin<a is formed by an 

 outpushing of the embryonic brain known as the primary optic vesicle, and the 

 lens is formed as an inpushing of the ectoderm of the side of the head. Now 

 these newt embryos are very tolerant of operations : it is perfectly possible to 

 slit open the skin and cut off the optic vesicle and yet the wound will heal up 

 and the embryo will survive, only in this case no lens will be formed by the 

 ectoderm on the operated side. But a still more wonderful experiment has been 

 performed. The amputated optic vesicle has been inserted under the skin in a 

 hinder region of the body : the wound has healed up, and the optic vesicle has 



