64 COMPARATIVE PHYSIOLOGY. 



shower of animal dust visible only by the highest powers of the 

 microscope, each particle of which proves to be the nucleus 

 from which a future individual arises. 



The study of reproduction thus establishes the conception of 

 a unity of method throughout the animal and, it may be added, 

 the vegetable kingdom, for reproduction in plants is in all main 

 points parallel to that process in animals. 



But why that costly loss of protoplasm by polar globules ? 

 For the present we shall only say that it appears necessary to 

 prevent parthenogenesis ; or at least to balance the share which 

 the male and female elements take in tlie work of producing a 

 new creature. It is to be remembered that both the male and 

 female lose. much in the process — blood, nervous energy, etc., in 

 the case of the female, while the male furnishes a thousand-fold 

 more cells than are used. But the period when organisms are 

 best fitted for reproduction is that during which they are also 

 most vigorous, and can best afford the drain on their super- 

 iluous energies. 



SEGMENTATION AND SUBSEQUENT CHANGES. 



After the changes described in the last chapter a new epoch 

 in the biological history of the ovum — now the oosperm (or fer- 

 tilized egg) — begins. A very distinct nucleus (segmentation 

 nucleus) again appears, and the cell assumes a circular outline. 

 The segmentation or division of the ovum into usually fairly 

 equal parts now commences. This process can be best watched 

 in the microscopic transparent ova of aquatic animals which 

 undergo perfect development up to a certain advanced stage 

 in the ordinary w^ater of the ocean, river, lake, etc., in which 

 the adult lives. 



Segmentation among invertebrates will be first studied, and 

 for this purpose an ovum in which the changes are of a direct 

 and uncomplicated nature will be chosen. 



The following figures and descriptions apply to a moUusk 

 {Elysia viridis). We distinguish in ova resting stages and 

 stages of activity. It is not, however, to be supposed that abso- 

 lute rest ever characterizes any living form, or that nothing is 

 transpiring because all seems quiet in these little biological 

 worlds ; for we have already seen reason for believing that life 

 and incessant molecular activity are inseparable. It may be 

 that, in the case of resting ova, changes of a more active char- 



