PREFACE. IX 



these terms might well be adopted in the Crustacea for the 

 three divisions of the alimentary canal. In the Phoronidea, 

 Bryozoa, and Brachiopoda, it is impossible to be so precise in 

 our terminology, for the origin and homology of the various 

 divisions of the alimentary canal in these forms are more or 

 less obscure. This is notably the case in the adult Bryozoon, 

 where it is quite impossible at present to interpret the parts 

 of the canal, all of which appear to arise from the ectoderm. 

 The use of the terms fore-, mid-, and hind-gut must not, 

 consequently, in this group be regarded as implying any 

 morphological homology with the similarly-named parts in the 

 Crustacea, but only an analogy with those parts. 



All through this work the genital glands are spoken of as 

 arising from, or in connection with, the mesoderrn ; and in 

 places even the splanchnic or somatic layer of the mesoderm 

 is specially mentioned as giving origin to the genital cells. 

 These statements arose from the incompleteness of our know- 

 ledge, at the time when this work was published, concerning 

 the early history of the genital rudiment. A number of 

 observations have, however, since been made on this point in 

 various groups of Invertebrate-, especially in different orders 

 of worms and Arthropoda, which tend to show that the 

 primitive genital cells are separated from the somatic portion 

 of the embryo at an extremely early period in the cleavage 

 of the egg. In fact, in some cases, directly after the first 

 cleavage plane has appeared, it can definitely be pointed out 

 which of the two spheres will give rise to the genital rudiment, 

 although this cell will also form certain other rudiments, the 

 actual isolation of the genital cell taking place somewhat 

 later (as early as the 32-celled stage in Cyclops, according to 

 Hacker). All these observations lend support to the belief 

 that the invisible genital rudiment is from the first quite 

 distinct from the somatic rudiment ; the former, at all events, 

 attains a visible distinction at a much earlier period than is 

 assigned to it in this work (see also Vol. i., p. 12, footnote). 



MAETIN F. WOODWAKD. 



ROYAL COLLEGE OF SCIENCE, LONDON. 

 December, 1898. 



