ECHINODERMS 89 



molecules, and he believes that their actions on the echinoderm egg are 

 related rather to their influence on the physical condition of the cytoplasm 

 than to any direct alteration of the chemical metabolism. 



Finally, a very unexpected point has recently been discovered by 

 Lrndahl (1953) v^ho has claimed that, in the two species of sea-urchin he 

 studied, the micromeres are haploid in chromosome number. It is not 

 clear how this remarkable example of nuclear differentiation (cf. p. 3 54) is 

 related to the high Vegetative' activity of these cells. 



As regards the localisation of the gradients, most authors agree that 

 they must be in the cortex. The argument is primarily the negative one. 



Figure 5.5 



Sections of early gastrulae stained with Janus Green, which is at first blue 

 (large dots) changing to red (fine dots) as it becomes reduced; in the un- 

 dotted areas the colour has faded altogether. A i, 2, 3, three stages over a 

 period of 50 mins. in an isolated vegetal half, showing the reduction spread- 

 ing from the most vegetative region. B i, 2, 3, a period of 55 mins. in an 

 isolated animal half, the reduction starting in the most animal region. C, an 

 animal half into which micromeres have been grafted, showing the induction 

 of an archenteron, with an associated region of rapid reduction, and the slow 

 progress of reduction in the neighbourhood of the apical tuft. D, micro- 

 meres were implanted on the left between an-i and an-2 of a normal 32-cell 

 stage, as in Figure 5.4 middle row; a region of rapid reduction has appeared 

 in their neighbourhood. (From Horstadius 1952.) 



