124 BULLETIN OF THE 



development of the egg, but without referring to gastrulation, or the 

 layers involved in the first polypide. 



Metschnikoff ('71, p. 508) and Nitsche (75, p. 349) maintain that 

 the outer layer of the embryonic " cystid " goes to form the inner layer 

 of the primitive polypides, and that its inner layer forms the outer 

 layer of the polypides. 



Eeinhard ('80% pp. 208-212) is more explicit concerning the early 

 stages than preceding authors. Apparently the egg segments regularly, 

 and undergoes embolic invagination. The blastopore closes. There is 

 a circular groove in the anterior part of the embryo (Barrois's mantle 

 cavity), and from the cap or "hood" which the mantle cavity sur- 

 rounds, the wall of the " cystid " or colony-wall is subsequently formed. 

 The embryo is already composed of three layers, " an outer, the tunica 

 muscularis, and the entoderm." All three layers of the " hood " share 

 in the formation of the polypides, but the fate of each layer is not 

 clearly described. 



Haddon ('83, p. 543) suggests that the gastrula is to be regarded as 

 one in which the alimentary tract is retarded in development, and that 

 the enlarged coelomic diverticula, such as occur in Sagitta, etc., line 

 nearly the whole of the so-called archenteron. From the small mass of 

 true entoderm at the pole opposite the blastopore the alimentary tract 

 arises. This suggestion, unfortunately, has no positive facts for its 

 support, and could be of service only upon the assumption that the 

 alimentary tract of the first polypide is formed from the inner layer of 

 the " cystid " ; but this assumption is contrary to the observation of all 

 who have written on this subject. 



Kraepelin ('86, p. 601) has also observed the "gastrulation," but he 

 believes that it is to be interpreted as the precocious formation of an 

 enterocoel, in which case the invagination to form the first polypide is 

 to be regarded as the true gastrulation, the inner layer of the cystid as 

 mesoderm, and the inner layer of the bud as entoderm. 



By far the most satisfactory and complete account of the embryology 

 of fresh-water Bryozoa is that of Korotneff, '89. The genera studied 

 were Alcyonella and Cristatella. Since the development takes place 

 inside of an ocecium, the use of the section method is necessary for the , 

 elucidation of the details of the embryological processes. Apparently 

 the egg segments regularly and forms a blastula. Loose cells are given 

 off from the inner surface at one pole of this blastula. These arrange 

 themselves in an epithelium, lying immediately inside of the ectoderm, 

 over a part only of its inner surface ; so that while the upper two-thirds 



