68 PRINCIPLES OF EMBRYOLOGY 



parthenogenetic eggs o£ Chaetopterus which 'differentiate without cleavage' 

 (p. 63), and in isolated and non-nucleated polar lobes of molluscs 

 (Morgan 1933). The operative agent in these instances is almost certainly 

 located in the egg cortex. 



There are many other instances in which it can be shov^oi that the local 

 properties of the cortex influence the course of the cleavage planes. This 

 factor is of importance in nearly all eggs in the formation of the small 

 polar bodies. Morgan (1937) has tried to discover why the maturation 

 spindles in the egg normally give rise to such extremely unequal divisions 

 of the cell body. He showed that in the marine snail Ilyanassa the second 

 polar-body spindle could be shifted into the middle of the egg by centri- 

 fuging, and that in this position, when it is no longer near the polar cortex, 

 it is capable of causing the egg to divide into more or less equal parts. It 

 only does so if the egg is still somewhat elongated after the centrifugation; 

 if the egg becomes completely rounded up again, a centrally-placed matur- 

 ation spindle fails to cause it to divide. In the parthenogenetically activated 

 Urechis eggs studied by Tyler (p. 54), the displaced polar-body spindles 

 seem to be more effective and able to cause an equal division even of a 

 spherical egg, provided they have been shifted away from the polar 

 cortex. 



Another clear example of the influence of the cortex is provided by the 

 experiments of Lehmann (1946) on the freshwater ohgochaete Tubifex. 

 In this form, considerable protuberances are pushed out from the body of 

 the cell, both at the first and second polar-body divisions and at the first 

 cleavage division (Fig. 4.5). In the polar-body divisions, which occur 

 with the spindle near the animal pole, the protuberances are arranged in a 

 more or less symmetrical manner around the animal-vegetative axis. 

 The first cleavage division is unequal and gives rise to a small AB cell and 

 a large CD cell. The protuberances at this division form mainly at the 

 equator of the ceU, in particular in the region of the ^B blastomere. They 

 are thus arranged bilaterally symmetrically as seen from the animal pole. 

 By moderate centrifugation the spindle of the second polar-body division 

 can be moved from its normal position without the structure of the cortex 

 being materially affected. If the egg is arranged so that the centrifugal force 

 is parallel to the animal-vegetative axis, the internal contents of the cell are 

 stratified and the polar-body spindle moved from the animal pole into the 

 interior. It is found that the pattern of the protuberances which form at the 

 next cell division is hardly altered, even if the spindle has been moved 

 right down to the vegetative pole of the egg. If, however, the egg is 

 orientated so that the centrifugal force is at right-angles to the axis, the 

 spindle is shifted towards the egg equator on one side. With tliis orienta- 



