38 



INVEETEBEATA 



CHAP. 



by a definite layer of dermal cells. This cavity is wedged in between 

 the layer of dermal spicules and a flagellated chamber. As it 

 enlarges to suit the size of the growing embryo, it encroaches on the 

 cavity of the flagellated chamber, since the layer of dermal spicules is 

 unyielding. (Dendy, 1889.) 



The first stages of development must therefore be studied in 

 transverse sections of the adult. 



When the larvae emerge they must be encouraged to settle on 

 some convenient portable object. If it is desired simply to make 

 whole mounts, the bottom of the vessel in 

 which the parents are contained is strewn 

 with coverslips, and these are removed when 

 the young sponges have attached themselves 

 to them, and immersed in 1 per cent solu- 

 tion of osmic acid tQl fixation is effected, 

 then stained in picro-carmine and mounted 

 whole. If it is desired to cut sections of the 

 larvae, the coverslips must be covered with 

 a layer of paraffin wax or photoxylin, which 

 can be scraped off and the larvae thus 

 removed, when they can be dealt with 

 by the methods described in the previous 

 chapter. 



The egg divides into two, four, and eight 

 blastomeres, which are arranged in one plane, 

 and, from the 4-cell stage, they surround a 

 central cavity open at both ends, which owes 

 its existence to their mutual separation 

 (Fig. 13). This stage is followed by a division 

 of all the cells into two tiers, so that sixteen 

 cells are formed in two rows, and then each 

 of these rows is subdivided into two further 

 rows, and so we reach the 32 -cell stage. 

 Divisions now follow one another in the 

 individual cells somewhat irregularly, and 

 thus an oval vesicle is constituted, which 

 may be termed the blastula, one pole of 

 which is rounded and one flattened, whilst inside it there is a cavity 

 which is a development of the cavity formed by the separation of the 

 first segments of the egg, and which is termed the blastocoele. 



One opening of this cavity to the exterior, that at the pointed 

 end, is by this time closed, but that on the flattened " basal " surface 

 persists for some time, though it too eventually closes. The cells 

 immediately surrounding this latter pore are distinguished from the rest 

 by becoming extremely granular. The granular cells increase by 

 division to thirty-two, whilst the remaining cells become extremely long 

 and columnar, and each develops a flagellum. The columnar half of the 

 embryo is pressed against the wall of the yielding chamber, but the 



Fig. 13. — Two stages in the 

 segmentation of the egg 

 of Sycandra raphanus, 

 (After Schulze. ) 



A, 8-cell stage in \rliich all the 

 blastomeres are in one tier with 

 a central aperture. B, 16 -cell 

 stage blastomeres arranged in 

 two tiers of eight each round a 

 central aperture. 



