IX. Genetic Relations to other Echinoderms. 1915 396 



as observed in the larva of the modern Asteroid, that forms the basis 

 both of MacBride's own hypothesis and of his criticism of the so-called 

 Pelmatozoic Theory. It may, however, be pointed out that in the 

 history of both the Echinoderm race and the Asteroid individual, the 

 hydrocircus begins as a hydrocoel crescent ; also that in the larval 

 Asteroid the stalk is on that side of the mouth where the crescent is 

 open, and is not within the circumoral nerve-ring. The passage of 

 the stalk in either direction would therefore be quite easy at a far 

 later period of race-history than that to which Professor MacBride 

 feels compelled to restrict it. 



Now the two remarkable features of Echinoderm morphology 

 which any hypothesis should seek to explain are, first, the hyper- 

 trophy of the left side with the correlated torsion, secondly, the 

 radiate (normally quinqueradiate) symmetry. 



The first of these, which affords the chief reason for the 

 Pelmatozoic Theory, is frankly left unexplained by Professor 

 MacBride. He writes : "It can only be described as an idiosyncrasy 

 of Echinoderms that bilateral symmetry is unstable, and that, 

 therefore, radial symmetry was arrived at by the overgrowth of the 

 organs of the left side, etc." (1914, Embryology, p. 562). Similarly 

 Professor Herouard in an interesting note (Jan., 1915, Bull. Inst. 

 Oceanogr. No. 301) describes this "idiosyncrasy" as a hemiplegia, 

 " un phenomene teratologique . . . dont nous constatons 1' apparition 

 sans malheureusement pouvoir en expliquer les causes." "On peut 

 dire, que la serie des etres [echinodermales] que nous considerons 

 comme representant revolution normale, n'est dans sa totalite qu'une 

 branche de la teratologic representant la serie des monstres nes viables 

 et capables de se reproduire." 



On the supposition that the tendency to this monstrous paralysis of 

 the right side arises from something in the larva itself, the only 

 explanation hitherto suggested has been that of Dr. Er. Meves (1912, 

 Arch. mikr. Anat., vol. 80, pp. 81-123). Having observed in 

 Parechinus miliaris that the middle-piece of the spermatozoon passes 

 at the first cleavage of the spermovum into one of the two blastomeres, 

 he supposes that the substance of the middle-piece is, in the course of 

 successive cleavages, eventually confined to that part of the pluteus 

 which becomes the sea-urchin, and that those parts of the pluteus 

 which disappear consist of cells which, in the course of cleavage, have 

 received none of the middle-piece substance. This hypothesis is 

 merely a succession of uncorroborated assumptions, and even if we 

 could for a moment accept the supposition that the male characters 

 (Vererbungspotenzen) were confined to the embryonic echinoderm, 

 the remaining cells of the larva being merely trophoblast, we should 

 not thereby explain the peculiar changes of symmetry. 



Reducing it to its simplest terms, Herouard (1915) describes the 

 change as the replacement of the original binary axis by a secondary 

 quinary axis. In Asteroidea and Echinoidea, he says, this quinary 

 axis is at right angles to the binary axis. In Ophiuroidea and 

 Holothurioidea, which both MacBride and Herouard regard as derived 

 respectively from Asteroidea and Echinoidea, the quinary axis is 

 inclined to the binary axis and ends by almost coinciding therewith. 



