4o8 PHYSIOLOGY OF THE INVERTEBRATA. 



plasm to the clay, we must compare the nucleus to the potter 

 that fashions it. The nucleus comprehends all the physio- 

 logical properties, the totality of which goes to constitute life." 

 Concerning the first mode of reproduction — viz., that of 

 transverse fission, Balbiani states, that in forty-two days 

 Paramcecium can produce 1,384,116 young, that is to say 

 that a single individual organism measuring 0.2 mm. long, 

 grows 277 metres in bulk.* 



The Porifera. 



Eeproduction takes place asexually — by fission and by 

 gemmation ; and sexually — by the production of spermatozoa 

 and ova. The needle-shaped spermatozoa lie in small pockets 

 lined with cells until required. The ova, derived from the 

 cells of the mesoderm, are naked amoeboid cells with a 

 germinal vesicle and spot. They are fertilised before leaving 

 the parent. The impregnated ovum divides into two, four, 

 eight, and more cells, and thereby passes into the morula 

 condition. The cells of the morula subsequently become 

 separated into two layers — an epiblast and a hypoblast. 

 These layers give rise respectively to the ectoderm and endo- 

 derm of the young animal. The embryo sponge is a free 

 swimming larva, and in such a condition it is stated to be in 

 the planula stage of its life-history. After a time the ciliated 

 cellular portion or hypoblast of the free swimming embryo 

 invaginates, and the dark granular cells or epiblast grow over 

 it. The latter form the ectoderm and the mesoderm is also 

 derived from them. The invaginated cells (ciliated) give 

 rise to the endoderm of the gastric cavity. This constitutes 

 the gastrula stage in the development of the Porifera. 

 After a time the young sponge becomes more or less cylin- 

 drical, and an osculum and inhalent pores are produced : and 

 calcareous spicules appear in the mesoderm. 



* For further information on the reproduction in the Infusoria, see 

 Mantegazza's Jiicerche sulla (jencrazione defjU lufu.sorii, e descrizione di 

 alcu'tie nuoce specie (1852) ; and W. Saville Kent's Manual of Infusoria. 



