I08 PRINCIPLES OF EMBRYOLOGY 



left. The region where the two nuclei fuse, and where the two crescents 

 are thickest, is in the eventual posterior and ventral part of the embryo, 

 while the opposite side of the egg is the anterior-dorsal. 



Shortly after fertilisation, the clear cytoplasm spreads over the greater 

 part of the animal half of the egg. The original central material of the 

 unfertilised egg, the grey yolky plasm, is now confined to the vegetative 

 region. It gradually becomes differentiated into a darker mass at the 

 vegetative pole, and a lighter grey material which forms a crescent at 

 the opposite side of the egg to the yellowish stuff. The egg thus comes to 

 have a fairly complicated architecture. 



These various regions of the egg are highly significant for the future 

 development. By careful observation, Conklin could follow each region 

 through the subsequent stages, and determine which organs it eventually 

 formed. The end-product which a given region of the egg (or early 

 embryo) will eventually form if left to itself in the intact egg is spoken 

 of as its 'prospective fate' (sometimes the expression 'presumptive fate', 

 derived from the German, is used). Conklin was thus in a position to 

 make a map of the early cleavage stages, marking on it the prospective 

 fate of the various parts ; in fact, in Styela, the map was more or less made 

 for him by the colouration of the various regions. This is not usually the 

 case. In Amphioxus, for example, although the general set-up is probably 



released a clear grey cytoplasm. The yellow cytoplasm is accumulating at 

 the vegetative pole. 



(f) The sperm nucleus s is visible in the yellow cytoplasm which lies at the 

 vegetative pole. The clear grey cytoplasm has moved down and lies just 

 above the yellow. 



{d) Shortly before the first cleavage. The sperm nucleus has moved up and 

 met the female nucleus just below the equator. The yellow and grey cyto- 

 plasms have moved up with the sperm nucleus. The yellow crescent is more 

 or less superficial, the grey one extends into the depth of the egg. 

 (e) Two-cell stage. The grey crescent is becoming more diffuse. 

 (/) Eight-cell stage from side. 



(g) Sixteen-cell stage from animal pole. Note the bilateral symmetry. 



(ii) Early gastrula from vegetative pole, looking into the wide open blasto- 

 pore b; on the dorsal side of this are the future neural cells n.p and chordal 

 cells c. 



()■) Longitudinal section through young larva, n.p. neural plate, c notochord, 

 e ectoderm, end endoderm lining the archenteron, m mesoderm, b blasto- 

 pore. (In this and Figures/, g and h the yellow crescent material is shaded so 

 that its movements can be followed but it carmot actually be recognised by 

 its colour so clearly as the diagram suggests.) 



(j) Longitudinal section through a young larva: c, notochord; ey., eye spot; 

 end., endoderm; t.e., endoderm of tail; mes., mesenchyme; «., nerve cord; 

 n.v. neural vesicle; pap., adhesive papilla. 



(After Conklin 1905.) 



