232 



THE CONTINUITY OF THE RACE 



Homolecithal eggs contain a very small amount of yolk, and this is 

 evenly distributed throughout the cytoplasm. Such eggs are very small 1 

 and undergo a markedly regular and symmetrical type of embryonic 

 development that ends with the production of a postembryonic larval 

 stage. This free-living larva must pass through a further period of de- 

 velopment before attaining the structure of the adult. The eggs of star- 

 fishes, sea urchins, and marine worms are of this type, and those of the 

 primitive chordate Amphioxus are very nearly so. 



Telolecithal eggs contain an abundant .store of yolk that is massed 

 toward one pole of the egg. The greater part of the cytoplasm is con- 

 centrated near the opposite ("animal") pole, with the nucleus near 

 its center. Telolecithal eggs vary widely in the amount of yolk they 



Fig. 15.12. Diagrammatic sections of eggs, showing differences in amount and distribution 

 of yolk. A, homolecithal egg of starfish. B, mildly telolecithal egg of frog. C, strongly telo- 

 lecithal egg of whitefish. D, centrolecithal egg of honeybee. The alecithal egg of mammals is 

 exemplified by the human egg shown in Fig. 16.4 These eggs are not drawn to scale. Their 

 relative sizes may be judged by the apparent size of the nucleus, which is actually about 

 the same size in all. (c) Cytoplasm; (n) nucleus; (y) yolk. (Modified from Turtox chart, 

 courtesy General Biological Supply House, Inc.) 



contain. The frog's egg, which is mildly telolecithal, has little more 

 than half of its bulk composed of yolk; the hen's egg (the "yolk" of the 

 hen's egg is the true egg) is strongly telolecithal, with the yolk com- 

 prising much more than 95 per cent of the whole egg and the cytoplasm 

 occupying a small superficial disk at the animal pole. Fish eggs are some- 

 what intermediate, although not much less strongly telolecithal than the 

 hen's or reptile's egg. 



Centrolecithal eggs are characterized by having a comparatively 

 large central core of yolk surrounded by a peripheral layer of cytoplasm. 

 The development of the centrolecithal egg is even more strongly modified 

 by its yolk than is that of the telolecithal egg. Insects produce eggs of 

 this type. 



Alecithal eggs. The eggs of all viviparous mammals contain little 

 if any yolk and superficially resemble homolecithal eggs. Their develop- 



1 Eggs of this type are usually correlated with a breeding habit that necessitates 

 the production of huge numbers of eggs. 



