3i8 THE UNFERTILISED EGG AS A [pt. m 



use a phrase of William Harvey's, "hid in obscurity and deep night". 

 It is as yet much too early to try to draw any conclusions from the 

 very fragmentary figures which are all that we have at our disposal, 

 and we may well admit that one of the most urgent needs of chemical 

 embryology is a much wider extension of our knowledge of the static 

 chemistry of the egg. This is a quite indispensable preliminary to the 

 investigation of the metabolism of the embryo in the lesser known 

 forms. The attempt has already once been made to link up in some 

 way the chemistry of the egg with what is known of the type of 

 embryonic development which takes place in it. Wetzel in 1907 

 analysed the eggs of a sea-urchin, a crab, a cephalopod, and an 

 elasmobranch fish. He pointed out that the eggs he studied 

 were examples of varying richness in yolk, of total and partial, 

 equal and unequal, superficial and discoidal cleavage, as well as 

 chemical systems. Taking the egg of Strongylocentrotus lividus as 

 his first case, he regarded it as typical of a class of alecithic 

 eggs, of a total and equal cleavage type, and he drew attention 

 to the fact that it was rich in water and in salts, but poor in 

 fatty substances, in nitrogen, and in phosphorus. Similarly, in the 

 case of the mollusca, where there is no very definite type of 

 development, the egg of Sepia could not stand as representative of 

 any wider class than the cephalopods, but, as far as it went, it 

 showed that the cephalopod egg was rich in nitrogen, poor in fat and 

 inorganic substances, with a moderate phosphorus and water-content. 

 The decapod Crustacea, to which Maia squinado belongs, have a 

 purely superficial type of cleavage, with no cell-multiplication in that 

 part of the egg which holds the yolk. Accordingly, the egg possessed 

 a moderate fat and water-content, a moderate ash, and much protein 

 and phosphorus. 



The mammalian ovum is still as unknown chemically as it was 

 when Wetzel was writing, and it may be found to have a 

 constitution not unlike the alecithic echinoderm eggs. For the 

 eggs of birds (and of reptiles, which only differ from them in 

 having very little egg-white) Wetzel found a low protein and 

 water-content, a high proportion of fat and ash, and a large amount 

 of calcium and phosphorus. Here cleavage would only take place 

 at one isolated point on the surface of the mass of food-material. 

 In the amphibia, the richness of yolk, while much more significant 

 than in lower classes, does not reach the level of birds and reptiles. 



