226 MR GEORGE BROOK ON THE 



only transitory, and later another connective tissue covering takes the place 

 of the primary one. 



The whole of the blood and connective tissue, in its widest sense, are 

 developed at a later period outside the region of the segmented blastoderm, and 

 are therefore parablastic in their origin. 



Thus the mesoblast in the ordinary sense (the middle germinal layer of 

 Remak) is a compound, and not a simple layer, and the two portions may be 

 spoken of as archiblastic and parablastic mesoblast. The parablastic mesoblast 

 of His almost exactly corresponds with the mesenchyme of the brothers 

 Hertwig (11). 



Finally, then, according to His, the parablast has nothing to do with 

 the formation of the primary germinal layers, but is utilised later to form 

 that portion of the mesoblast which gives rise to the blood and connective 

 tissue series. 



More recently Waldeyer (26) has contributed a most important paper on 

 the subject, which, I take it, goes to the root of the matter. Waldeyer calls 

 attention to the structure of a typical meroblastic ovum, and to the relative 

 distribution of protoplasm and yolk. The yolk is passive food material, which 

 can only be utilised by the embryo after assimilation. Beneath the germinal 

 disc there are a number of protoplasmic processes (Keimfortsatze) which press 

 in amongst the passive food material ; and there is also a thin cortical film of 

 protoplasm around the yolk. Segmentation takes place in the germinal disc, 

 but does not affect the protoplasmic processes or the cortical layer. Later 

 nuclei appear in the protoplasm, which is as yet unsegmented, and not in the 

 yolk itself. The cells thus produced give rise to parablastic tissues. Thus the 

 parablast layer is derived from the original protoplasm of the ovum, and not 

 from white yolk cells, and its nuclei are also derivatives of the first segmenta- 

 tion nucleus. Waldeyer distinguishes a primary segmentation, resulting in the 

 formation of the archiblast, and a secondary segmentation, which frequently 

 takes the form of budding, by which the parablastic tissues are derived. 

 Waldeyer also points out that there is no essential difference between mero- 

 blastic and holoblastic eggs ; but that throughout the animal kingdom a 

 graduated series of modifications in the segmentation process are to be noticed, 

 which are largely due to the varying quantity of passive food material contained 

 within the ovum. It is also certain that the unequal distribution of the yolk is 

 as important as its quantity in bringing about modifications in the segmentation 

 process. According to Waldeyer's view, the formation of parablastic elements 

 in holoblastic eggs is more easily explained than on His's view. During the 

 segmentation process division takes place most rapidly in that part of the ovum 

 containing least food yolk. At the base of the vegetative pole those cells are 

 found which contain most yolk, and therefore segment more slowly. Those 



