Il6 BIOLOGICAL LECTURES. 



plexity, owing partly to the interaction of the developing or- 

 ganism with its environment, partly to the multiplication and 

 interaction of its own parts. If this view be correct adult 

 homologies need not necessarily preexist in the form of egg- 

 homologies but may be created as the ontogeny progresses. 



How such a process is possible may be illustrated by one or 

 two cases. Loeb has shown that the color pattern in the yolk- 

 sac of a fish-embryo {Fiindnlns) is not in itself predetermined, 

 but depends on the distribution of the blood-vessels. The pig- 

 ment-cells are at first uniformly distributed, but upon the 

 establishment of the circulation of the yolk-sac they migrate 

 towards the vessels (probably, as Loeb suggests, attracted by a 

 chemical substance in the blood) and thus give rise to a defi- 

 nite pattern. Graf has recently shown, ^ in like manner, that 

 the color-patterns of leeches are not in themselves inherited 

 but depend upon the arrangement of the muscle-fibres, be- 

 tween which the amoeboid pigment-cells wander. In either of 

 these cases the assumption of a special set of "determinants," 

 etc., for the color pattern, is absurd. 



A third illustration, of the most 'instructive kind, is the case 

 of the ciliated arms of the Phiteus larva of sea-urchins which 

 has been carefully studied and discussed by Herbst.^ As is 

 well-known, these organs are definite in form and number and 

 have a characteristic arrangement ; and no one would question 

 that the arms of the various species may be homologized with 

 one another. Each arm contains a calcareous axis or spicular 

 skeleton, and in the developing larva the arm grows out as the 

 axis is formed. If now the larvae be made to develop in water 

 containing no calcareous matter (Pouchet and Chabry) or in 

 water containing a small excess of potassium chloride (Herbst) 

 no spicules are formed and in consequence no arms are produced. 

 Thus arises a larva (Diagram III) closely similar in general ap- 

 pearance to a Tornaria. In this case it is quite unnecessary 

 to suppose that the ectoderm inherits any tendency to produce 

 a definite number of arms in a particular position. The for- 



1 Reported at the meeting of the Am. Morphological Society in December, 

 1894. 



2 Zeit. tviss. Zool., LV. 



