408 TRANSACTIONS OF SECTION D. 



an embryo to the purposeful endeavours of a man who is bent on building a 

 house of a particular type and who takes whatever materials he can lay his hands 

 on in order to effect his object. Now certainly purposeful endeavour is the most 

 familiar of all the activities which we see around us, and there is therefore 

 nothing wrong in Driesch selecting this most familiar of all phenomena in order 

 to throw light on the development of the germ. The great objection to it is, I 

 think, that it is unfruitful : it does not enable us to compare one kind of 

 development with another. For we simply have to instal a different kind of 

 entelechy with a different purpose in every kind of egg, and there the matter 

 ends. On the other hand, there are records of phenomena, rapidly increasing in 

 number with the extension of research, of which 'we can only give a rational 

 account by postulating some form of the hypothesis of organ-forming sub- 

 stances : for in some cases these substances are actually visible to the naked eye 

 in the living egg. We shall give a short account of the most striking instance 

 of this, viz. the developriient of the egg of the Ascidian Cynthia partita as 

 described by Conklin. This egg before fertilisation contains the usual large 

 bladder-like nucleus or germinal vesicle characteristic of immature eggs. The 

 mass of the egg consists of a cytoplasm rendered a slaty-grey colour by 

 inclusions of yolk, but in its outermost zone are included many particles of a 

 bright yellow pigment. When the egg ripens the germinal vesicle bursts and 

 the clear fluid which it contains spreads out in a cap at one pole of the egg. 

 Now, fertilisation takes place and the spermatozoon enters the egg at the 

 opposite pole from that at Avhich this cap of clear matter is situated. The 

 effect of this entry — long before the male pronucleus has reached the female 

 pronucleus — is as if the egg were struck by a whirlwind. All the yellow 

 particles of pigment are sucked downwards towards the entering spermatozoon 

 and so is the original clear substance. The female pronucleus descends from 

 the upper pole to the centre of the egg, where it meets the male pronucleus and 

 the yellow and clear substances form two concentric crescents around the 

 posterior half of the egg. When segmentation of the egg begins and the egg 

 divides into two, the yellow mass is likewise divided into two, and each half 

 receives an inclusion from the yolky cytoplasm which becomes incorporated 

 with it. Thereafter, during the subsequent stages of development, the clear, 

 blue and yellow cytoplasms remain distinct from one another and as cell- 

 division progresses they become gradually confined to definite cells. Then it 

 becomes evident that the clear substance forms the ectoderm, the blue stuff the 

 endoderm, whilst the yellow stuff forms the mesoderm and in particular the 

 longitudinal muscles which flank the Ascidian tadpole's tail. That these sub- 

 stances are in reality essential for the formation of the organs in which they are 

 situated is shown by the fact that when one of the first four cells is killed, and 

 thereby one half of the yellow substance removed, the resulting tadpole has 

 muscles only on one side of the tail. That the arrangement of these substances 

 in the egg is due to some attractive influence radiating from the male pro- 

 nucleus is proved by the circumstance that when an egg is entered by two 

 spermatozoa the yellow material forms two crescents, one embracing each male 

 pronucleus. Another most interesting conclusion to be drawn from the study of 

 this development is that the separation of these substances corresponds to the 

 DIFFERENTIATION OF THE GERMINAL LAYERS about which SO much dispute has raged, 

 and that the cutting up of the developing organism into cells is a phenomenon 

 of secondary importance. For we find that both notochord and nerve cord arise 

 from the same group of cells, termed by Conklin the chorda-neural cells : 

 but this is not to be interpreted as meaning that these organs were differentiated 

 out of a common ancestral organ, because when these chorda-neural cells are 

 closely examined they are found to include within themselves areas of both the 

 clear and blue cytoplasms, and when they divide the clear and the blue regions 

 are assigned to different daughter-cells, and the clear daughter-cells give rise to 

 the nerve-tube whilst the blue daughter-cells grow into the notochord. We find 

 in this an additional confirmation of Hertwig's view that the nuclei are all alike 

 and endowed with all the potentialities of the organism, and that it is the 

 cytoplasmic areas which become unlike each other. Of course Driesch may 

 reply that the organ-forming substances are merely the conditions and not the 

 effective causes of organ-formation. Putting aside the obvious retort that the 



