EMBRYOLOGY 1 77 



living things. Formative forces, polarity, symmetry, and purposeful regula- 

 tions are examples of this in the embryological realm. 



The story of genetics has -become so interwoven with that of experi- 

 mental embryology that the two can now to some extent be told as a 

 single story. It is true there are still wanting many important points of 

 contact, but enough is known to make it possible to attempt to weave 

 them together into a single narrative. Although each has developed in 

 large part independently of the other, nevertheless today their interde- 

 pendence is so obvious that the geneticist takes for granted the main out- 

 lines of the facts of embryology, and the embryologist is coming to realize 

 his dependence on the evidence from genetics. For example, cell division 

 and the behavior of the chromosomes at maturation of the eggs and sperm 

 have supplied the working scheme for the theory of heredity. The 

 changes that take place during the maturation of eggs and sperm are con- 

 tributions from embryology. Conversely, genetic analysis has made it pos- 

 sible to go behind these visible changes into the very constitution of the 

 chromosomes themselves. The common meeting point of embryology and 

 genetics is found in the relation between the hereditary units in the chromo- 

 somes, the genes, and the protoplasm of the cell where the influence of the 

 genes comes to visible expression. Concerning the manner of functioning 

 of the genes during development, I have contrasted, in the following pages 

 whenever an opportunity arises, two possible views, and suggested a third. 

 The implication in most genetic interpretation is that all the genes are 

 acting all the time in the same way. This would leave unexplained why 

 some cells of the embryo develop in one way, some in another, if the genes 

 are the only agents in the results. An alternative view would be to assume 

 that different batteries of genes come into action as development proceeds. 

 The former view, namely, that all the genes are acting all the time in the 

 same way, leaves the embryological problem where it has always been 

 supposed to be, viz., in the protoplasm. The alternative view might appear 

 to give a formal explanation of development, but is inconsistent with re- 

 sults obtained by changing the sequence of the cleavage planes by com- 

 pression. Roux and Weismann attempted to explain development in some- 

 what this way, by assuming that the determinants in the chromosomes are 

 quahtatively sorted out during development. There was at the time no 

 evidence in favor of this view, and there is now much that is opposed to 

 it. The idea that different sets of genes come into action at different times 

 is exposed to serious criticism, unless some reason can be given for the time 

 relation of their unfolding. 



The following suggestion may meet these objections. It is known that 

 the protoplasm of different parts of the egg is somewhat different, and 

 that the differences become more conspicuous as the cleavage proceeds, 

 owing to the movements of materials that then take place. From the proto- 

 plasm are derived the materials for the growth of the chromatin and for 



