EiMBRYONIC DEVELOPiMENT. 15 



shaped processes, from the free ends of which tlie embryos are developed like 

 huds and eventually cut off. In quite young ovicells, on the contrary, a single 

 egg-cell, surrounded by a follicle, is found ; this appears to give rise to the 

 finger-shaped Inidding organ mentioned above as producing the embrj-os. In 

 Crista, therefore, the number of embryos produced by the early division of the 

 primary embryo is larger. The mature larvae swim out through tlie tubular 

 aperture of the ovicell. 



According to Van Beneden and Peruens, the maturation of tlie egg is 

 connected in a certain regular way with the disintegration of the polypidc* that 

 produces it, and, in some forms {Flustra truncata, Microporella malusii, Bugv/a 

 simplex and turhinata) with its later regeneration, so that, when the egg is fully 

 mature, the polypide undergoes histolysis, and becomes changed into a bro^\■u 

 body. While the ovary brings another egg to maturity, a new polypide forms. 

 For details of these processes of regeneration see below, p. 55. Among these 

 must also be reckoned the above observations of PiiouHO on Alcyonidiiim duplex. 

 In the Phylactolaemata also, as a rule, during the development of the embryos 

 and the statoblasts, the polypide to which they belong degenerates, but, in this 

 case, there is no subsequent regeneration. 



11. Embryonic Development. 



The mature, sjDherical, or ellipsoidal eggs are surrounded by a 

 hyaline membrane called by Pergens the chorion. In the ovum 

 can be recognised a vesicular nucleus with spherical nuclear bodies, 

 and a granular yolk often yellow or brown in colour. The two polar 

 bodies, Avhich are usually of unequal size, correspond in position to 

 the animal pole of the egg. 



The first ontogenetic j^rocesses in the egg of the marine Ectoprocta 

 are best known in Lepralia (Barrois, Nos. 6 and 1), in Tendra 

 zostericola and Bowerhanlcia (Kepiachoff, Nos. 32 and 34), in BiKjuIa 

 calathus (Vigelius, No. 39), and in Microporella mahmi (Pergens 

 No. 27). In these eggs cleavage is total and almost equal (Fig. 6). 

 The two-celled stage is attained Ijy means of a meridional furrow, 

 and the four-celled by means of another meridional furrow at right 



* The expressions " polypide " and " cystid '' correspond to an older view, 

 according to which the cystid forming the wall of a chamber represents an 

 individual which gives rise asexually through budding to the polypide. Each 

 chamber of the Bryozoan stock, consisting of a polypide and a cystid, would 

 then represent a double individual or a miniature colony. This view was 

 founded on the great independence of the polypides as shown in the processes 

 of degeneration and of regeneration above-mentioned. Althongli we do not 

 share this view, we still retain in use these expressions which luive becohie 

 established. The cystid, then, means to us the lower part of the body- wall, 

 while the polypide rejn'esents the retractile anterior section of the body with the 

 intestinal canal attached to it. These two are merely parts of one individual. 



[Acting on the suggestion of Dr. Harmeu, we have sulistituted the term 

 " zooecium " for cystid in referring to the outer parts of the ordinary adult 

 individual, as this term is of much more general use in English works on the 

 Bryozoa. — Ed.] 



