BIRDS' EGGS 267 



the navel closes up ; the allantois shrivels ; and the young 

 bird breaks open the doors of its prison. 



Development implies a realising or actualising of what is 

 implicit or potential in the inheritance. It means a cashing 

 of the legacy. This is not possible unless certain conditions 

 be fulfilled — a certain amount of oxygen, a certain degree of 

 warmth and humidity. The conditions of development 

 form the normal " nurture," and peculiarities in the nurture 

 will influence the expression of the inheritance. Thus Miss 

 Florence M. Alsop has shown (19 19) that the development 

 of the nervous system of the chick is affected by changes 

 of temperature. It is quickened by raised temperature and 

 retarded by lowered temperature, while excessive tempera- 

 tures on the plus side induce a large percentage of abnormali- 

 ties, either in the brain or in the spinal cord. 



It is not very unusual to find chick embryos showing 

 abnormalities. One of the commonest is some form of 

 duplicity. Sometimes this occurs near the beginning, for 

 the blastoderm (or disc of embryonic cells) may show four 

 primitive streaks instead of the normal one. Sometimes 

 there is an almost complete dupHcation of structures on one 

 blastoderm. Sometimes there may be a common head- 

 region and a duplicated embryonic body, and it is very 

 difficult to draw the line between this and cases where the 

 living chick has an extra pair of hind legs. It may seem far- 

 fetched to regard an extra pair of hind legs as comparable 

 to a twin, but the gradations are many. In several cases we 

 have seen that the presence of an extra pair of hind-legs is 

 associated with the presence of an extra caecum at the 

 junction of the small intestine with the large, enough of itself 

 to show that the duplication is more than superficial. There 

 are cases, however, where a duplication of a limb is of a 

 different order, being due to some pressure on the pri- 

 mordium of the limb, which involved a splitting into two 

 independently developing parts. 



We speak of the vita minima of the embryo and that is 

 indeed true ; it is often a flickering flame which a gust may 

 blow out altogether. On the other hand, there is often a 



