ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 959 



/3. Polyzoa. 



Development of Polyzoa.* — In the study of the development of 

 Bugula calathus Norm. Dr. W. J. Vigelius has made an important 

 contribution to our still incomplete knowledge of the ontogeny of the 

 Polyzoa. After discussing the specific characters of B. calathus and 

 the technique employed in his investigation, Dr. Vigelius passes to 

 consider — 



The maturation of the ovum. It seems probable that the ovary is a 

 product of the mesenchymatous parenchyma, arising as a local pro- 

 liferation of indifferent cells. It is attached or appressed to the parietal 

 layer of the neural wall of the sexual individual. A few incipient ova 

 (described in detail) are surrounded by small flattened follicular cells. 

 A struggle for existence begins, and the potential ova are reduced to 

 one, or rarely to two. The ovary becomes in the meantime free ; a 

 follicular remnant probably remains to form the rudiment of a new 

 ovary. The egg becoming independent wanders through the body- 

 cavity to the ovicell ; it is then likely that fertilization occurs. 



The brood-capsule arises somewhat later than the ovary in the form 

 of two diverticula from the free distal wall of the sexual individual. 

 The growth of these two sacs, and the manner in which they unite are 

 carefully described. The interior is lined by a continuation of the 

 parietal layer of the parenchyma. In one region, where the embryo is 

 afterwards borne, a layer of peculiar cylindrical cells is developed. 

 They seem to be aided by wandering cellular elements, probably from 

 the mesenchym, which apparently discharge an important formative 

 function. The elongated parenchymal cells which form two bundles 

 of ovicell muscles are then described. 



The segmentation is alecithal. The first plane of division lies in 

 the short axis and cuts the animal and vegetative pole. The second is 

 also meridional in the long axis of the ovum, crossing the former. 

 The third is an equatorial plane, at right angles to the two previous. 

 The differences in size between the segments are neither very marked 

 nor yet constant. In the stage with 8 segments a small but distinct 

 blastocoel can be detected. The 16 stage arises as the result of a 

 double division in two planes, which lie on either side of, and 

 parallel to the first meridional division ; and that with 32 results 

 from a division similarly related to the second meridional plane. The 

 blastosphere is then distinct, and the oral and aboral halves are clearly 

 distinguished. The shape of the young embryo varies considerably, 

 now like a biconvex lens and again ellipsoidal. 



The formation of the germinal layers. In sections of embryo at the 

 last-mentioned stage, four cells are seen within the blastocoel, closely 

 adjacent to the outer cells, about the centre of the oral surface. These 

 form the rudiment of the endoderm, and Vigelius gives a number of 

 reasons which make it probable that they are intruded in consequence 

 of an epibole. They gradually increase in number, and form a 

 complex of cells completely filling the blastocoel. This mass remains 

 during the further embryonic development very passive, and represents, 



* MT. Zool. Stat. Neapcl, vi. (1S8C) pp. 499-541 (2 pis.). 



