378 SUMMARY OF CUERENT . EESEARCHES RELATING TO 



the causes of differentiation reside in the first instance in specific organ- 

 forming cytoplasmic materials. 



The third lecture deals with differentiation. 1, "Experiment shows 

 that the removal of certain parts of the cytoplasm of the ovum entails 

 the absence or at least the defective development of certain organs of 

 the embryo or larva. Hence there are in the cytoplasm certain material 

 factors on which the formation of certain characters depends. These 

 characters are part of the total inheritance. 2. Every visible substance 

 in the cytoplasm is not, however, necessarily such an organ-forming 

 substance, as experiments with the centrifuge demonstrate. 3. Experi- 

 ments on heterogeneous hybridization indicate that it is the large 

 characters — those of the phylum, class, order, family to which the 

 animal belongs — that are carried by the cytoplasm, and, this means, 

 transmitted through the female germ-cell alone. 4. At the same time 

 the cytoplasm is, during prematuration stages, indebted to the nucleus 

 for certain elements in its structure. In the female, therefore, these 

 nuclear elements of the cytoplasm are concerned in the transmission of 

 inheritable characters, as well as the chromosomes. 5. In the chromo- 

 somes the germ-cells of the two sexes are alike, and these chromosomes 

 are certainly concerned in the transmission of some characters. 6. It is 

 known («) by observation, and (b) by experiment that there are 

 qualitative differences between individual chromosomes, and that a 

 complete set of these different chromosomes must be possessed by every 

 cell in the body if development is to be normal. It is further probable 

 that the chromosomes are heterogeneous. 7. The different activities of 

 the different chromatic elements are probably only called forth by 

 differences in their environment, that is, in the cytoplasm to which they 

 are distributed. It is known that differential behaviour of nuclei can 

 be incited by cytoplasmic dissimilarity. 8. Hybridization experiments 

 on nearly related forms make it certain that smaller characters — generic, 

 specific, varietal, and individual — can be transmitted as easily from the 

 father as the mother, and therefore through the nuclei." 



The volume ends with a discussion of the interaction of parts upon 

 one another in development. Thus under certain conditions a lens may 

 be formed over the optic vesicle from cells other than those that are usually 

 devoted to its formation. The auditory vesicle may be transplanted 

 into another individual and become surrounded by a capsule developed 

 from connective tissue which would otherwise have had a very different 

 fate. The directive stimulus of one part upon another is a factor of 

 the utmost importance, and one that simplifies the whole process of 

 development. 



Sex of Tadpoles Reared from Artificially Parthenogenetic Ova.* 

 J. Bronte Gatenby experimented with five thousand eggs of Rana 

 temporaria smeared with blood and lymph and pricked with a fine glass 

 needle under the usual precautions. He succeeded in raising about 

 fifty tadpoles to the closure of the neural folds. Fifteen were brought 

 on to the stage when the external gills become covered by the epidermal 



* Quart. Journ. Micr. Sci., ii. (1917) pp. 213-16 (5 figs.). 



