314 THE UNFERTILISED EGG AS A [pt. iii 



this connection the monotreme egg would be a chemical study of 

 great interest, and it is characteristic of the exasperating fragmenta- 

 tion of this field of work that all we know about the monotreme egg 

 is that its membrane seems to have the properties of a keratin. The 

 suggestion that the metabolism of the fowl, operating on a con- 

 tinuously high level of energy turnover, would naturally tend to fill 

 up the eggs with fat, and is associated with the well-known higher 

 temperature of the avian body (Wetmore) may not be without value, 

 but any special emphasis on fat metabolism in adult birds is precluded 

 by the statements in Schulz's review. 



It is very significant that as animals became more complicated and 

 more adaptable to varied surroundings, higher, in fact, in the taxo- 

 nomic scale, they loaded their eggs to a greater extent with yolk. Since 

 the extra material was usually fatty acids, this process appears 

 strikingly in Table 31 . The effects of the yolk have long been familiar 

 to embryologists, and have been best described, perhaps, in a passage 

 by Milnes-Marshall. "The immediate effect of a large amount of 

 yolk", he said, "is to retard mechanically the processes of develop- 

 ment, but the ultimate result is to shorten them. This paradox is 

 readily explained. A small egg, such as that of Amphioxus, starts its 

 development rapidly, and in about eighteen hours gives rise to a 

 free-swimming larva, capable of independent existence, with a 

 digestive cavity and a nervous system already formed ; while a large 

 egg such as that of the hen, hampered by the great mass of yolk 

 by which it is distended, has, in the same time, made very little 

 progress. From this time onwards, however, other considerations 

 begin to tell. Amphioxus has been able to make this rapid start owing 

 to its relative freedom from yolk, but now this freedom becomes a 

 retarding influence, for the larva, containing within itself but a very 

 scanty supply of nourishment, must devote much of its energies to 

 hunting for and to digesting, its food, and hence its further develop- 

 ment will proceed more slowly. The chick embryo on the other hand 

 has an abundant supply of food in the egg itself and has no occasion, 

 therefore, to spend its time searching for it, but can devote its whole 

 energies to the further stages of its development. Hence, except in 

 the earliest stages, the chick develops more rapidly than Amphioxus 

 and attains its adult form in a much shorter time. The tendency of 

 abundant yolk to lead to shortening or omission of the ancestral 

 history, is well known. The embryo of forms well provided with yolk 



