SPIRALLY CLEAVING EGGS 99 



Perfectly normal embryos are formed iiA, JB or C quadrants are elimin- 

 ated, and this necessitates some replacement of them by converting part 

 of the D substance for the purpose. Similarly the macromere 4D can be 

 dispensed with. Even if the individual somatoblasts are killed at the stage 

 v^^hen the germ-bands are beginning to form, some regulation is still 

 possible. For instance, if the mother-cells of the two ectoderm bands are 

 killed, the embryo has at first no ectoderm, but some is later formed by a 

 conversion of mesoderm cells. If the mesoderm mother cells are killed 

 the mesoderm is not produced from any other element; but it is found 

 that there are characteristic defects in the development of the uninjured 

 ectoderm, which points to the existence of essential influences of meso- 

 derm on ectoderm in normal development (Penners 1938). Thus the 

 mesoderm seems to have in some sense a leading role in the whole 

 embryogenesis ; we shall find a much more striking example of the same 

 thing in vertebrates. 



The essential substance of the D quadrant can sometimes be seen, in 

 the form of a special type of cytoplasm. In the mollusc Dentalium, for 

 instance, the cleavage is very oddly modified in comiection with a region 

 of clear cytoplasm lying near the vegetative pole. Before the first division, 

 this material is pushed out from the egg in a broad pseudopodium-like 

 lobe. The cleavage plane runs in such a way that the whole of this gets 

 into one of the two daughter-cells ; when the lobe has been retracted and 

 the whole cell rounded up, this blastomere is considerably larger than the 

 other. A similar process occurs in the succeeding division, and most of 

 the material of the lobe eventually gets into the D blastomere (some may 

 be included in C). One of the early and classical experiments on spiral 

 eggs is that of E. B. Wilson, who showed that when the polar lobe is 

 removed the development of the whole embryonic region (derived 

 from id and 4 J) is suppressed (Fig. 6.4). Perhaps more surprising is a recent 

 result (Novikov 1940, on Sabellaria); by treatment with KCl, the first 

 cleavage can be made to become equal, so that the polar lobe substance 

 get into both the first two blastomeres instead of only into one. Twin 

 embryos are formed. This must involve a considerible amount of regula- 

 tion, so here the lobe material has acted, not merely as a region of the 

 egg whose fate has been determined precociously, but as one which can 

 initiate embryonic development by the cells surrounding it. Other agents 

 (e.g. abnormal temperatures, anaerobiosis, etc.), may upset the position 

 of the first cleavage spindle in many types of spirally cleaving eggs, with 

 the result that twins are produced (Tyler 1930). 



We therefore seem to be confronted with the situation that, in the 

 so-called mosaic eggs, there is at a very early stage (which varies from 



