THE DEVELOPMENT OF ORGANISMS 717 



Echidna, Ornithorhynchus, and ant-eater have lasted well so far; 

 thanks doubtless to fresh adaptations, so obvious indeed in the 

 last named of these. 



But our point here is that in an approach to the general fact of 

 individual development, it is important to keep in mind its relation 

 to racial evolution, of which it is in some measure a condensed 

 recapitulation, especially as regards the great steps in organ-forming. 



THE EGG-CELL OR OVUM.— In some animals, as we have 

 noticed in a previous chapter, there is asexual multiplication by 

 buds or by separated-off pieces of the body; occasionally parthe- 

 nogenesis occurs, in which the offspring has a mother but no father; 

 but with these exceptions every multicellular animal begins its 



MA 



Fig. 107. 



Dimorphism of Sex-Cells. MA, a macrogamete, comparable to an egg-cell, 

 showing the nucleus (N) and vacuoles (V). MI, a microgamete, comparable 

 to a sperm-cell. 



life as an egg-cell with which a male cell or spermatozoon has 

 entered into intimate union in fertilisation. 



The ovum has the usual characteristics of a cell; its cytoplasm is 

 a complex colloidal mixture of substances; its nucleus or "germinal 

 vesicle" contains a definite number of rodlets or loops or otherwise 

 shaped bodies (chromosomes) which carry the "genes" or hereditary 

 initiatives; there may be distinct inclusions (e.g. mitochondria) in 

 the cytoplasm; there is often a store of reserve material or yolk, 

 and this may be relatively large in amount; and round the whole 

 there is a delicate cell-membrane representing a cell-wall, often 

 added to extrinsically so that it becomes a multicellular envelope, 

 and often protected inside a non-living shell, familiarly calcareous 

 in birds, chitinous in insects, and horny in skates and dogfish. 



The size of the ovum varies enorm^ously in different types, but 

 the differences are more due to the amount of yolk than to the 



