93 



We have ao long been accustomed to the conception that the 

 egg consists of an upper ectodermal region and a lower entodermal 

 region that this comes to some extent as a surprise. The peripheral 

 laj'cr of protoplasm is present also in the eggs of Selachii, other 

 Meroblastica and in Mammalia. But further work is necessary 

 with regard to these, and even in the case of Holoblastica before a 

 general statement can be made as to the nature and importance 

 of the layer. 



The Organisation of the Teleostean Egg. — The foregoing 

 has already indicated the main features of the organisation of the 

 Teleostean egg. The protoplasm is arranged b}^ a streaming pro- 

 cess into a polar mass containing the nucleus, a peripheral layer, 

 and a reticulum containing the greater quantity of the yolk. De- 

 velopment shoA^s that the latter, together with a thin film of pro- 

 toplasm around it, forms the basis of the entoderm of the embryo 

 and of the yolk sac. The rest of the protoplasm of the polar cap 

 and of the peripheral layer forms the ectoderm. The nuclei neces- 

 sarily are confined primarily to the polar mass, and secondarily^ by 

 the multipUcation at the blastoporal margin invade the region of 

 the peripheral protoplasm, converting the latter into ectoderm and 

 entoderm. 



The polar mass is only resolved into ectoderm and entoderm. 

 At the margin when the growth of the blastopore commences, and 

 out with the area of the polar mass, the notochord and mesoderm 

 are developed between the ectoderm and entoderm. The polar 

 massjis therefore a mesoderm-free area, which by the separation 

 of the ectoderm and entoderm presents a cavity — which may be 

 called a segmentation cavity — although a discussion of the morpho- 

 logical features of this cavity is not called for here. 



It will be plain from the above and a consideration of the 

 diagrams illustrating the development of the whiting (page 84) 

 that the margin of the blastopore is resolved into a median (dorsal) 

 region, the structures of which enter into the formation of the 

 embryo proper, the rest being concerned in enveloping the yolk. 

 The ectoderm of the latter is part and parcel of the ectoderm of 

 the embryo, the entoderm forms the entoderm of the yolk sac, and 

 is finally absorbed with the yolk. 



