1!)0 SUMMARY OF CURRENT RESEARCHES RELATING TO 



in which segmentation generally commences ; (2) of a thin film of cor- 

 tical protoplasm entirely surrounding the yolk, and frequently presenting 

 a considerable dilatation at tho yolk-polo ; (3) of a number of filamentous 

 protoplasmic processes, mainly confined to the base of the germinal area, 

 which serve to keep up a communication between the latter and the more 

 purely nutritive yolk ; (4) of the nutritive yolk itself, which constitutes 

 the greater portion of the ovum. 



Segmentation. — No nucleus was observed until after the third furrow 

 was formed. The first furrow which takes an equatorial direction is 

 the third of the series. There is a period of quiescence between the 

 completion of one furrow and the commencement of the next. On the 

 formation of the third furrow there are two distinct layers, of archiblast 

 which goes on segmenting, and of parablast which remains as a con- 

 necting area between the latter and the yolk. The mesoblastic seg- 

 mentation is compared with the holoblastic division of the frog ovum, 

 and the nutritive physiology of the embryo at this stage is discussed. 



Parablast. — Mr. Brook gives an historical resume of the various 

 opinions held in regard to the origin and relations of the parablast. He 

 then communicates his own observation. At the end of the primary 

 segmentation-stage in the herring, the parablast, which has increased 

 considerably at the expense of the yolk, leaves the periphery, and collects 

 mainly under the archiblast. The archiblast becomes differentiated into 

 two layers. The outer and somewhat flattened cells form the epidermal 

 layer of epiblast. The cells more centrally situated are larger, more 

 rounded, less deeply stainable, loosely aggregated, and represent the 

 nervous layer of the epiblast. Beneath the archiblast, the parablast 

 appears as a thick layer of protoplasm undergoing division into cells. No 

 karyokinesis was seen. Brook regards the cells thus formed as secondary 

 segmentation products in the sense of Waldeyer. They join those 

 derived from primary segmentation in the archiblast, and soon become 

 undistinguishable from them. In eggs forty-five hours after fertilization 

 the subgerminal parablast is only a very thin film. The peripheral 

 parablast, however, is a thick wedge-shaped mass, stretching from the 

 base of the morula to the equator of the egg, and containing a consider- 

 able number of rows of nuclei. All this is before the extension of the 

 morula over the yolk, before the formation of the segmentation-cavity, 

 and before the differentiation of the germinal layers. In most Teleostean 

 ova the morula is solely archiblastic, and it is only later that parablastic 

 elements are utilized ; but here, at least, two distinct batches of para- 

 blast-cells are budded off and unite with those of the archiblast before 

 any trace of differentiation of the morula. The final morula is parablastic 

 as well as archiblastic. The difference is important, and probably con- 

 nected with the early elaboration of the parablast, and probably also 

 with the absence of a vitelline circulation. When the segmentation- 

 cavity appears, the parablast forms its floor, the cells of the morula its 

 roof and sides. The thickening which forms the commencement of the 

 blastodermic rim may be in part due to the addition or segregation of 

 cells from the parablast. The parablast certainly extends under the 

 thickened peripheral portion of the blastoderm, and around its margin 

 forms a thickened welt. At a later stage it is distinctly evident that 

 the primitive hypoblast is mainly, if not entirely, formed from the para- 

 blast. Mr. Brook defends this conclusion against objections. Some 

 later embryonic stages are briefly alluded to. 



