204 CONK LIN. [Vol. XIII. 



an explanation Biology, as a causal science, is bound to seek 

 after and expect. It is only against that narrow and near- 

 sighted view which mistakes aims for achievements and which 

 would explain all the mysteries of development by such known 

 forces and conditions as gravity, surface tension, cohesion, 

 viscosity, and the like that exception is here taken. It is safe 

 to assert that, before any such explanation can be given, our 

 conception of mechanics in general must be greatly enlarged. 

 The mechanical explanation of vital phenomena is a great task, 

 and one not to be accomplished in a year or a century. We 

 ought not to deceive ourselves by supposing that we have 

 already reached, or are indeed near, such an explanation. 



University of Pennsylvania, 

 March, 1896. 



Supplementary Note. 



Renewed study of Figs. 56, 62, 65-73 renders it probable that the interpretation 

 of certain cells in those figures, as shown in the plates and explained in the text, 

 is erroneous. I have already called attention to the fact that, at the time of the 

 formation of the fifth' quartette (Fig. 54), the entire egg becomes irregular and 

 many landmarks disappear. Owing to this fact it is extremely difficult to follow 

 the lineage through this period. I have all along been aware of certain dis- 

 crepancies in the interpretation of some of the figures mentioned but supposed 

 that this might be due to variations in the time of division and in the size of 

 resulting cells ; e.g., in Fig. 53 there are, in the anterior arm of the cross, eight 

 cells arranged in two rows, four in a row. In Fig. 56 there are apparently only 

 six cells, three in each row. In Fig. 62 the number is indefinite, though only the 

 basals could be plainly recognized. 



I am now inclined to the view that the large paired cells marked V in Figs. 

 62, 69, 70, and] 7 1, and ib^-^-^-^-^-^ in Figs. 65, 66, 67, 68, 72, and 73 are the same 

 and that they are identical with the middle cells of the anterior arm (ib'-^-^-'-^ and 

 j^i.2.2.2.2^ Fig, 50). Accordingly in all these cases the cells lying apical to these 

 middle cells are the i7i7ier and outer basals. Fig. 53. Throughout all these figures 

 the inner basals remain well marked, the outer ones, however, undergo division, 

 forming in Figs. 69, 70, and 71 two large and two small cells (ib'-^-°'''^ and 2b'-'-'). 

 The latter, which should be labelled ib'-^-^-^-^ are probably thrown away, the 

 former remain as the narrow cells (ib'-^-^-^-'. Figs. 65, 66, 67, 68) just apical to 

 the middle cells while on the apical side of these are the inner basals. In Fig. 62 

 the four small cells lying between the inner basals (ib'-^-'-'-', etc.) and the middles 

 (V) are probably the derivatives of the outer basals, no one of which has yet 

 been thrown away. According to this interpretation, the first velar row runs through 

 the tip cells of the anterior arm just as it does through the tip cells of the right and 

 left arms, while a portion of the outer basals of the anterior arm, and ttot the tip cells, 

 is thrown away. 



