EMBRYONIC DEVELOPMENT. 15 



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

 Imds 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 budding organ mentioned above as producing the embryos. In 

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

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

 aperture of the ovicell. 



According to Van Beneden and Pergens, the maturation of the egg is 

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

 produces it, and, in some forms (Flustra truncata, Microporella malusii, Bugula 

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

 mature, the polypide undergoes histolysis, and becomes changed into a brown 

 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 Protjho on Alcyonidium 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. 



II. Embryonic Development. 



The mature, spherical, 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, which are usually of unequal size, correspond in position to 

 the animal pole of the egg. 



The first ontogenetic processes in the egg of the marine Ectoprocta 

 are best known in Lepralia (Barrois, Nos. 6 and 7), in Tendra 

 zostericola and Boicerbankia (Kepiachoff, Nos. 32 and 34), in Bugula 

 calathus (Vigelius, No. 39), and in Micropoi*ella malusii (Pergens 

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

 The two-celled stage is attained by 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. Although we do not 

 share this view, we still retain in use these expressions which have become 

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

 while the polypide represents 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. Harmer, we have substituted 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 tin? 

 Bryozoa. — Ed.] 



