GENERAL INTRODUCTION. xxv 



a 



animal, which is then said to be monoecious or hermaphrodite ; or it 

 may be lodged in another animal of the same species, in that case said 

 to be dioecious or of separate sexes. In hermaphrodite animals the 

 testis may ripen at a different time to the ovary, a phenomenon known 

 as successive hermaphroditism, and in most instances certainly a safe- 

 guard against self-impregnation, e. g. in the hermaphrodite Gastropoda. 

 Some hermaphrodites, however, are self-impregnating, such as Cestoda, 

 some Trematoda. The Nematode genus Angiostomum is a unique example 

 of an organism, which though anatomically a female, is yet a self- 

 impregnating hermaphrodite. The actual process of impregnation is, 

 briefly stated, first the penetration of the spermatozoon into the ovum, 

 either through its envelope at any spot, or by a special aperture, the 

 micropyle, secondly the fusion of the protoplasm of the two cells, which 

 is perhaps an unessential feature, followed thirdly by fusion of the nuclei, 

 often termed the male (spermatozoal) and female (ovular) pronuclei. The 

 two pronuclei approach each other, and the granules of the surrounding 

 protoplasm are arranged round each of them, so as to form a star or aster 

 with a pronucleus as a centre. This aster is most pronounced on the 

 aspects of the pronuclei turned to one another. The ovum has now 

 become an oosperm, and it speedily undergoes fission or segmentation 

 and gastrulation. 



What is generally considered, but perhaps wrongly, to be the most 

 primitive mode of segmentation is seen in an oosperm, which is alecithal, 

 i. e. devoid, or nearly so, of food-yolk. The nucleus divides with mitosis, 

 and a constriction splits the oosperm into two equal or sub-equal halves 

 or blastomeres. Each half then divides again into two, and so on. The 

 two first divisions take place in a vertical plane, the third in a horizontal, 

 the fourth in a vertical, and the fifth in a horizontal, and then regularity 

 is lost. In many instances, however, a regular sequence of stages is not 

 recognisable. The result of segmentation is the formation of a hollow 

 sphere, the blastula or blastosphere, the cells or blastomeres being disposed 

 in a single layer round a central cavity filled with an albuminous liquid, 

 the blastocoele or segmentation cavity. In many instances the blasto- 

 coele is absent, or nearly so, and the term morula is then used instead 



polar globule. Such a value can hardly be assigned to the various forms of blastophores, nucleated 

 and non-nucleated, seen in spermatogenesis. The male cell is incapable of further development, 

 i. e. of parthenogenesis in animals where it is highly specialised ; so, too, in the higher plants, but 

 in certain of the lower male parthenogenesis appears to occur, see Vines, ' Physiology of Plants,' 

 p. 674 ; cf. Weismann, ' Continuitat des Keimplasma's,' Jena, 1885, cap. ii. p. 70, on the significance 

 of polar bodies, and cap. iii. p. 88, on the essential character of parthenogenesis ; Id. Nature, xxxvi. 

 1887. p. 607. 



