222 MR GEORGE BROOK ON THE 



fertilisation a morula mass of cells has been produced, which is represented in 

 fig. 12. It will be seen from the figure that the cells of the archiblast have 

 already become differentiated into two well-marked groups. The outer row of 

 cells are elongated and flattened, forming an epithelioid layer, whereas the 

 remainder are comparatively large round cells, which are only loosely aggre- 

 gated together. During the time that segmentation has been progressing in 

 the archiblast the cortical protoplasm has increased considerably in bulk. In 

 sections it may be seen that this layer gradually forms a thickening at the yolk 

 pole. When the morula stage is reached the cortical protoplasm leaves the 

 yolk pole, and gradually collects around and under the segmented disc. Even 

 at a considerably earlier stage the cortical protoplasm may be seen to be 

 accumulating towards the disc (see fig. 13, PI. XIV.); but after the morula stage 

 is reached there is no longer any protoplasm to be observed at the yolk pole in 

 the living egg. In stained sections there is always a thin film of protoplasm to 

 be seen around the yolk spheres, and this is increased from time to time by the 

 assimilation of more yolk. 



The collection of unsegmented protoplasm will be spoken of as the parablast. 

 As will be seen later, the part played by what I term archiblast and parablast 

 in the herring ovum is not identical with that which has been described by His 

 and others in other forms, but I conceive the same terms to be applicable. 



The Part played by the Parablast. 



Historical. — The mode of origin of the parablast, and the part which it 

 plays in the economy of the embryo, has during the past few years been one of 

 the most keenly contested problems in this branch of embryology. In 1868 

 His brought forward his well-known theory as to the development of the tissues 

 in meroblastic ova (12). He held that in the chick the whole of the tissues of 

 the future embryo were not derived from the three-layered blastoderm. That 

 the blood and connective tissue series of structures arise independently of the 

 segmented disc, and take their origin from the white yolk substance imme- 

 diately underlying the blastoderm and outside the embryonic area. In their 

 mode of origin the former set of tissues are known as archiblastic, and the 

 latter as parallactic. According to His, the nuclei of the parablastic cells 

 are derived from the white yolk spheres, which themselves have the value of 

 cells. The segmented disc supplies the material for the three germinal layers, 

 and the cells from the parablast find their way in between the cells of these 

 layers. In the same year Kupffer (18) described in Teleostean fish ova a layer 

 of protoplasm outside the germinal disc, in which, at the close of segmentation, 

 concentric rows of free nuclei make their appearance. His investigations were 

 made on Gasterosteus aculeatus, and other forms, and the following short 

 extracts from his paper show the position taken up at the time : — " Man sieht 



