684 PATTERNS AND PROBLEMS OF DEVELOPMENT 



SYMMETRIES AND ASYMMETRIES IN OTHER INVERTEBRATES 



Concerning origins or relations to any particular factors of develop- 

 mental pattern in triclads, rhabdocoels, trematodes, cestodes, and bryozoa 

 practically nothing is known; in the polyembryonic bryozoa the prob- 

 lem appears in particularly interesting form (pp. 536-37). As regards most 

 other invertebrates, we know little more than that patterns of symmetry 

 or asymmetry appear. In monembryonic insects polar pattern is prob- 

 ably derived from the ovarian tubule, but whether a ventrodorsal pat- 

 tern is present in the tubule is not known. How patterns of the definitive 

 embryos of polyembryonic insects originate is a problem for the future. 

 Concerning possible relations of symmetry patterns of other arthropods 

 to any particular factors, there seems to be no definite information. 



AMPHIBIAN DORSIVENTRALITY 



Among the eggs of vertebrates those of amphibians have been most 

 studied with reference to the problem of origin of dorsiventrality. Eggs 



of most amphibians show no definite evi- 

 dence of dorsiventrality preceding fertili- 

 zation. In various species a part of the pig- 

 mented surface adjoining the unpigmented 

 basal region becomes less deeply pig- 

 mented after fertilization, forming the so- 

 called "gray crescent" (Fig. 220). The 

 breadth of the crescent decreases bilateral- 

 ly from its broadest median region; this 

 region becomes the median dorsal region 

 Fig. 22o.~Diagrammatic outline of the embryo, the median plane passing 

 of frog egg viewed laterally with ap- through it and the apicobasal axis. The 

 proximate extent of one arm of gray crescent varies in extent and distinct- 



crescent mdicateu by broken Imes. o ^ • i i 



ness in different species, bemg most clearly 

 defined in certain anura, indistinguishable in some forms. Its median re- 

 gion is usually more distinct than lateral parts, and it is not sharply 

 bounded but shades off into the more deeply pigmented and the unpig- 

 mented regions; its visible characteristics suggest a gradient system.^^ 



Before the gray crescent was recognized as a visible feature of the 

 dorsal region, attempts had been made to determine whether a relation 

 between point of entrance of sperm or its path in the egg and the median 



'7 For accounts of the formation and characteristics of the crescent see Vogt, 19266, 19286; 

 Banki, 1927; Weigmann, 1927; also the general account by Schleip, 1929, pp. 55'^-^-''- 



