70 ORIGIN OF POLARITY, SYMMETRY, AND ASYMMETRY 



Cephalopod Molluscs, where the egg is not only polarised, but 

 bilaterally symmetrical in shape while still in the ovary. 



It is of considerable theoretical importance to find that one and 

 the same determination can occur either before or after fertilisation 

 in different forms. In Fucus, no axes of symmetry at all are deter- 

 mined until after fertilisation. In most animals, radial symmetry 

 and the primary axis are determined before fertilisation, bilateral 

 symmetry at or after fertilisation. In some insects and Cephalo- 

 pods, the determination of bilateral symmetry too has been shifted 

 back to the period before fertilisation, and takes place under the 

 influence of ovarian factors. It will be seen later that a similar 

 shift in time-relations has occurred as regards the processes of 

 chemo-differentiation in a number of forms, and that this shift 

 contributes to the difference, which long puzzled experimental 

 embryologists, between so-called "regulation-eggs" and ''mosaic- 

 eggs" (see Chap, v).^ 



§3 

 A further problem of symmetry is the determination of bilateral 

 asymmetry. There are certain animals in which nothing is known 

 as to the embryological determination of asymmetry, e.g. the skulls 

 of certain whales and owls with asymmetrical formation of some of 

 the bones ; the bill of the wrybill plover ; the various insects with 

 spiral torsion of the genitalia;^ the fish Anableps in which the 

 copulatory tube points either left or right in males, and the genital 

 aperture faces right or left in females ; ^ the flatfish, in which either 

 the left or the right side becomes uppermost when the fish is lying 

 on the sea bottom, and the structure of the head is modified ac- 

 cordingly ; or Amphioxus, in which the larval stages are markedly 

 asymmetrical. 



^ In his large book on experimental embryology, Schleip (p. 842) argues at 

 some length against the idea that the primary axes of the egg are imposed from 

 without, and supposes that they arise by self-differentiation, though they may 

 be modified by external agencies. It is logically almost impossible to conceive 

 how a non-polarised fragment of living matter can acquire polarity by self- 

 differentiation; and further, the experimental evidence in certain cases strongly 

 supports the view that external differentials are responsible (Fiicus, egg; Cory- 

 viorpha, redifferentiation). The concept advanced above, that very slight external 

 differentials may serve to release the capacity of the egg to develop polarity, re- 

 conciles both views. The type of the polarity is predetermined and therefore 

 "self-differentiating"; the direction of the polarity is determined from without. 



2 Richards, 1927. ^ Garman, 1895, 1896. 



