34 



BRYOZOA ECTOPROCTA. 



the statements of Davenport, Braem, and Kraepelin, invariably 

 corresponds to that pole of the embryo which is turned towards 

 the wall of the zooecium. At the same point, at a later time, the 

 first rudiment of the polypide is said to develop, so that we have 

 here an important contrast to the Gymnolaemata, in which, so far 



as we know, the primary 

 rudiment of the polypide 

 always develops at the 

 aboral pole of the larva. 



While the ingrowing 

 cell-mass more and more 

 fills the blastocoele, a 

 new cavity develops with- 

 in the former; analogy 

 with other animals might 

 incline us to consider this 

 cavity as the primary 

 enteric cavity, but we are 

 led by the study of the 

 further course of develop- 

 ment to regard it as the 

 body-cavity. The ingrow- 

 ing cell-mass represents the 

 mesoderm, and the cavity 

 that develops within it 

 the coelom. We thus 

 find here a gastrulation 

 in which the entoderm 

 proper is apparently want- 

 ing. As the coelomic cavity increases in size, the mesodermal 

 epithelial layer becomes closely pressed against the ectoderm, and 

 in this way a bilaminar, vesicular embryo develops (Fig. 16, C). 



'-£ 



Fig. 16. — Three stages in the embryonic development 

 of Plumatella (after Kraepelin). A, blastula stage 

 in the ooecium. B, stage of ingrowth of the meso- 

 derm. C, bilaminar stage, e, embryo in the blastula 

 stage ; ec, invaginated ectodermal part of the ooecium 

 (cf. Pig. 15) ; o, ooecium. 



We shall only be able to interpret this stage correctly if we compare it with 

 the stage that follows the gastrula in most marine Ectoprocta (Fig. 6, G, p). 

 The primary entoderm there yields the so-called central tissue which represents 

 the enteric rudiment of the larva, out of which, however, important mesodermal 

 organs of the primary zooecium are also derived. In the embryo of the 

 Phylactolaemata this tissue is represented by the inner epithelial layer. We 

 must here assume that the larval intestine has undergone excessive reduction, 

 so that not a vestige of it can be seen, and, taking into consideration the further 

 development, we must regard this inner layer as mesoderm, and the central 

 cavity enclosed by it as the coelom. 



