INVERTEBRATA, CRYPTOGAMIA, MICROSCOPY, ETC. 279 



then consisting of an ovoidal body formed of a solid mass of endoderm 

 cells, covered, except at one pole, by columnar, ciliated, ectoderm cells ; 

 the uncovered endoderm cells form the yolk-plug, and are at first not 

 ciliated. 



Soon the outer layer of endoderm cells begins to be distinguished 

 from the rest, and forms the foundation of the mesoderm, the central 

 mass of cells being now the true endoderm. The process of mesoderm- 

 formation begins in the neighbourhood of the blastopore. The meso- 

 derm cells are soon distinguished by the development of fusiform 

 siliceous spicules. 



Arrived at this stage, the embryo makes its way, by its somewhat 

 pointed anterior end, through its capsule into one of the canals of the 

 mother, and out by one of the dermal ostia. After the birth of all 

 the embryos, of which a single female Challnula may produce 100, 

 the parent is completely disintegrated, as a result of the mechanical 

 strain, so that each female can only produce a single brood. 



After birth the yolk-plug projects considerably, and its cells 

 develop cilia, but subsequently lose them again. In an aquarium the 

 embryos, like the swarm-spores of some algfe, always seek the side 

 turned away from the light. 



One remarkable fact about the free larva is that at its liinder end 

 a depression with raised edges is formed ; probably this represents 

 a rudimentary archenteron. 



The larva usually swims freely for about two or three days. Then 

 the cilia disappear from the posterior end and the larva appears 

 suddenly to become flattened out in a direction at right angles to its 

 long axis. The ectoderm then loses its cilia, is converted into a highly 

 contractile layer of flattened cells, and also grows round and covers 

 in the previously exposed endoderm cells of the hinder end. 



At the end of the third day certain endoderm cells in the interior 

 unite themselves into groups, become strongly pigmented, and form the 

 foundations of the ciliated chambers, a cavity appearing in them sub- 

 sequently. The gastric cavity is formed on the fourth day by a separa- 

 tion (Auseinanderweichcn) of the central endoderm cells. 



Sponfjes and the Germ-lamella Theory. — Keller considers it proved 

 that the Chaliniila possesses the tliree characteristic germ-layers ; that 

 it goes through an amphigastriila stage not preceded by an amphi- 

 blastula ; tliat it possesses a true Rusconian anus tilled up by the 

 yoke-i)lug ; tliat tlie mesoderm arises from the endoderm, the process 

 beginning all round the Rusconian anus, that is, in a radially 

 symmetrical manner. 



Tndiiidualifij of Sponges, — The autlior considers that " the sponges 

 are true Cculenterates, higher from a histological point of view than 

 most Hydromedusa), since tliey possess a true middle germ-layer. In 

 their histological relaticjus they are nutst nearly related to the corals." 



Formation of Free-swimming Buds by a Halisarca. * — Professor 

 F. E. Scliulze adds to our knowledge of the difficult question of the 

 development of sponges, l)y his observations on some fi-eo embryos 



* 'Zool. Anzeigor,' ii. (1870) p. 636. 



