DEVELOPMENT 201 



the cells, on further divisions, proceed, perforce, to arrange 

 themselves as a plate, and not as a little hollow ball. Their 

 relative positions are thus altered, so that if each cell had an 

 inevitable fate an abnormal embryo would result. Nevertheless 

 on release of the constraint the embryo effects a regulation so that 

 normal development proceeds. 



(3) In the sea-urchin development the blastula is a small, hollow 

 ball the wall of which consists of about 1,000 cells arranged in 

 a single layer. Here again the various regions of the embryo are 

 normally destined to give rise to different larval parts. It is 

 possible to cut the blastula in two hemispherical halves and, of 

 course, it is entirely a matter of chance how the cut is made : 

 obviously it may be made in very many different ways. Yet 

 each half-blastula effects a regulation and proceeds to form a 

 perfect, but dwarf gastrula. 



In these examples any of the equipotential cells of the embryo 

 behave as if what they are going to do depends on what the other 

 cells are doing. 



(4) When the frog egg has divided into two blastomeres it is 

 possible to kill one of the latter by puncturing it with a hot 

 needle. In such a case the other, uninjured, cell goes on develop- 

 ing but forms a half -gastrula. Apparently the power of regulation 

 fails in such a case. But the half-gastrula may absorb the dead 

 blastomere and then proceed to regenerate the missing embryonic 

 part. And in other variants of the experiment the power of 

 regulation can otherwise be shown. 



726. Isotropic and Anisotropic Ova. There are very many 

 such experimental results and they are often apparently contradic- 

 tory. But the contradictions largely clear up when we see that 

 the process of development may have proceeded long before 

 segmentation of the ovum begins. In many invertebrate eggs 

 there are differently coloured parts, zones, crescents, etc., of the 

 cytoplasm showing that the latter is already differentiated chemic- 

 ally : these we call anisotropic ova and we note that when 

 segmentation begins the different parts of the cytoplasm become 

 located in different blastomeres. This differentiation imposes 

 limitations on the embryogeny and it happens that mutilation 

 of such a segmented; anisotropic egg leads to the development of 

 a defective embryo. 



There will be a stage in the development of such an ovum 



