234 MR GEORGE BROOK ON THE 



mesenchym, and the blood. In many Arthropods they likewise supply 

 the entire ectoblast. 



4. In Elasmobranchs they play only a subordinate part in the formation of the 

 ectoblast, and must here probably be regarded as homologous with the 

 vegetative pole of holoblastic eggs. 



."). The merocytes arise with the first equatorial division of the segmenting 

 ovum. The germinal disc which represents the animal pole goes on 

 segmenting. The merocytes increase only to a trifling extent during 

 this period. The blastula cavity appears between the morula (archi- 

 blast) and the superficial layer of yolk charged with merocytes — that is 

 to say, between the animal and the vegetative poles of the egg. Its roof 

 gives rise to the ectoblast ; its floor supplies the ectoblast in the follow- 

 ing manner : — The embryonal cells formed from the merocytes are 

 pushed up into the blastula cavity from the yolk, and close its lumen. 

 This process commences all around the periphery of the disc, but is later 

 mainly confined to the posterior position (embryonal shield). The blood 

 and mesenchym cells also arise from merocytes. 



It will thus be seen that, according to Ruckert, the hypoblast is derived 

 mainly from merocytes and not from a rearrangement of the " lower layer cells " 

 of the primary segmentation, as has been maintained by previous authors. The 

 merocytes of Ruckert undoubtedly corresponds with the free nuclei in the 

 layer which I have termed parablast, and indeed in the Trout there is an 

 approach to the structure described by Ruckert. 



If these observations are correct, it will be necessary to modify considerably 

 our ideas of a meroblastic egg. Although meroblastic ova are usually regarded 

 as having been derived from holoblastic ones by the inclusion of an excess of 

 passive food yolk, the two appear to be more closely related than has generally 

 been admitted. We have been in the habit of regarding segmentation as only 

 taking place in the germinal disc, and have usually derived the three primary 

 germinal layers as a result of this segmentation process. The yolk is therefore 

 in the main regarded as a passive food store, which is assimilated later through 

 the digestive and circulatory systems. Waldeyer's idea of the interrelation of 

 archiblast and parablast certainly modifies this view, and, I take it, is a step in 

 the right direction. According to the views here advocated, a meroblastic egg 

 produces the germinal layers from both animal and vegetative poles of the 

 ovum, as is the case in holoblastic ova, but the means by which this end is 

 attained is different in extreme cases. As Ruckert has, however, pointed out, 

 there are a large number of types, particularly amongst the Arthropods and 

 Mollusca, which, so to speak, bridge over the gap between the two extremes. 

 The difficulty lies in the proper understanding of the vegetative pole in mero- 



