THE VICTORIAN NATURALIST. 73 



We commenced with the question, Are sponges animals or 

 plants ? and our examination of the circulatory and digestive 

 system indicates their animal status. But the most convincing 

 evidence, placing them, without doubt, amongst the animals, 

 comes from a study of their development. In this they are 

 distinctly holozoic. That we may learn a few facts about the 

 general principles of sponge development it will be well to take 

 as a type, Sycon raphanus, because it is easily obtained, and, in 

 the summer season, is usually well provided with embryos in every 

 stage of development. The following stages may be noted: — 



(i.) Reproductive cells are formed from cells in the mesoderm 

 of the sponge and the reproductive elements — male and female — 

 are fashioned from them. The male elements or spermatozoa are 

 formed by the division of these reproductive cells into a large 

 number of parts each of which develops a head and tail, whilst 

 the female cells or ova become somewhat spherical in shape and 

 are furnished with a large nucleus. 



(2.) The ova of one sponge are probably fertilized by the sperma- 

 tozoa from another and then commence to develop in the mesoderm 

 of the mother sponge near the walls of the flagellate chambers. 



(3.) Each ovum begins to divide in an orderly manner into 2, 

 4, and 8 parts, and so on — the cells at the upper pole being some- 

 what columnar in shape whilst those at the lower pole are spherical 

 and granular. (PI. I., A. 1, 2, 3, 4.) 



(4.) A little later and these layers are clearly marked off from 

 each other with a cavity between them and the blastosphere stage 

 is reached. (PI. I., A. 5.) 



(5.) A little later and the granular cells become pushed in 

 towards the columnar cells. This is probably due to the position 

 in which the embryo is lying — the granular cells being bounded by 

 spicules whilst the columnar cells are free to expand. This is 

 known as the pseudo-gastrula stage. (PI. I., A. 6, 7.) 



(6.) About this time the embryo bursts through the wall of the 

 flagellate chamber : the granular cells are pushed out again and 

 the embryo has the form of a hollow sphere (PI. I. B. and B. 1.) 

 The columnar cells now produce cilia and the embryo, by their 

 means, is free to move in the water. This is known as the amphi- 

 blastula stage. (PI. I., B. 2.) 



(7.) True invagination now takes place, the columnar cells 

 being pushed in whilst the granular cells are arranged in a single 

 layer outside and the segmentation cavity reduced to a mere slit. 

 This is the gastrula stage. (PI. I., B. 3.) 



(8.) A little later and the embryo attaches itself by its open end 

 to some foreign object. The outer granular cells become flattened, 

 the columnar cells lose their cilia and a gelatinous layer is pro- 

 duced between the other layers. Three distinct layers are, 

 therefore, now present — ectoderm and endoderm with a layer of 

 mesoderm between. (PL I., B. 4.) 



