SPIRALLY CLEAVING EGGS 97 



The cells derived from 2d form ectoderm, and overlie those from 4d 

 v^hich form mesoderm. Thus two bands are produced, one on the right 

 and one on the left, both lying on the surface of a mass of cells derived 

 from the A, B and C quadrants, and each consisting of a core of meso- 

 derm covered by ectoderm. The mother-cells continue to divide, and as 

 these bands elongate, they push out over the surface until they meet and 

 fuse; from the fused bands the adult worm develops. Thus in these eggs, 

 the whole embryonic portion is formed from 2d and 4^, the rest of the 

 cells taking little part. 



A very large number of experiments have been made on spirally 

 cleaving eggs, but rather little insight has been gained into the factors 

 which control their development. In general, it is found that from a very 

 early stage isolated blastomeres behave as though they were still part of a 

 complete egg, and develop only into those organs which they would 

 have formed if left undisturbed (Review: Schleip 1929). This fact gave 

 rise to the suggestion that there is, in these eggs, a strict localisation of 

 'organ-forming areas', or regions each of which could develop only into 

 certain definite organs. The egg was considered to be a mosaic of such 

 areas, and such 'mosaic' development was contrasted with the 'regulation' 

 development found for instance in echinoderms. 



We now know that the mosaic character is by no means absolute. As an 

 example which demonstrates both the truth and the falsity of the idea, we 

 may consider the egg of the nemertean Cerehratulus. Horstadius (1937) 

 has made some isolations and recombinations of the various rings of cells 

 at the i6-cell stage, quite comparable to his experiments on echinoderms 

 described on p. 85. In Cerehratulus he found completely 'mosaic' behav- 

 iour; each ring of cells, when isolated, formed only what would be 

 expected of it, and in combinations there was no sign of interaction be- 

 tween the animal and vegetative groups. But if one goes back to a slightly 

 earlier stage, things are not quite the same. Any nucleated fragment cut 

 off from the unfertilised egg, can, after fertilisation, develop into a normal 

 embryo. And even though the differences along the animal-vegetative 

 axis are fixed as early as the 8-cell stage, those in the other plane are 

 certainly labile as late as the four cell, since the isolated first four blasto- 

 meres may each give a normal larva. Thus there must be some process of 

 determination which gradually fixes the manner in which the parts of the 

 egg can develop; but, since this is complete by the 8-cell stage, it must go 

 much faster than in echinoderms, where we saw that regulation is possible 

 considerably later; and unfortunately in the mosaic eggs we know of 

 nothing which controls development in a way comparable with the ani- 

 mal and vegetative gradients (Fig. 6.3). 



