SPONGES 65 



(b) Gemmule Formation. This method of reproduction, though 

 occurring also in many marine sponges of various groups (Topsent), 

 is seen in its most typical form in Spongillinae, where its details 

 have been carefully studied (see especially Zykoff [33]), and which 

 may therefore be taken as a type of gemmule reproduction. 



The gemmules are formed in the late autumn as a protection 

 against the winter in Europe, but in the tropics they are more 

 usually formed at the commencement of the dry season, during 

 which the sponge is liable to desiccation. Each gemmule consists 

 essentially of a local aggregation of wandering cells, that is to say, 

 of trophocytes which become laden with refringent granules repre- 

 senting reserve material of the nature of food -yolk. A great 

 number of such cells, which may be termed statocytes, migrate by 

 their own activity into one spot in the skeletogenous parenchyma. 

 The cells of the parenchyma then secrete round them an adventitious 

 capsule forming the gemmule envelope (Fig. 56, A, B, and C, i.ch.e). 

 The fully formed gemmule is a tough, seed-like body, and consists 

 of a densely packed mass of statocytes surrounded by a special cap- 

 sule. Each statocyte resembles in appearance a blastomere of a 

 segmenting ovum ; its large vesicular nucleus can scarcely be made 

 out in the midst of the yolk granules with which the cells are 

 crammed (Fig. 56, (J). In the simplest cases the capsule may con- 

 sist merely of a chitinous membrane ; this may, however, be forti- 

 fied by the addition of a layer of spicules, which may be either the 

 ordinary microscleres of the parent sponge, as in Spongilla, or may 

 be composed of special spicules not found ordinarily in the sponge, 

 as in the case of the amphidiscs of Ephydatia (Fig. 56, ampli). 



The ripe gemmule is very resistent to vicissitudes of moisture 

 and temperature, and in Europe remains dormant until the spring, 

 the rest of the sponge dying away. The gemmules can be separated 

 from the parent sponge, and then give rise each on germination to a 

 tiny sponge individual ; but in nature they seem more often to 

 remain entangled in the skeleton of the parent organism, and to 

 repeople it, as it were, on the approach of warmer weather, so that 

 the sponges seem to die in the autumn and revive again in the 

 spring. On germination the capsule bursts and the contents creep 

 out, forming an irregular amoeboid mass. The statocytes multiply 

 actively and become tissue cells of various kinds. The finer details 

 of the process of cell differentiation remain to be accurately studied, 

 but would appear to resemble in all essential points the transforma- 

 tion of the blastomeres into tissue cells during the embryonic de- 

 velopment. In fact, the gemmule is physiologically equivalent to the ovum 

 at the dose of segmentation, i.e. to a mass of blastomeres enclosed in a 

 special capsule, and capable each of developing into one or another 

 form of tissue cell, with the difference, however, that the statocytes 

 are not derived like blastomeres from the segmentation of one 



