134 PATTERNS AND PROBLEMS OF DEVELOPMENT 



stages are uncertain as regards presence of a differential. In the early 

 echinoid blastula the basipetal reduction gradient becomes more distinct 

 (Fig. 45). At all levels reduction is most rapid in the inner ends of the 

 cells and progresses outward; this perhaps indicates merely that oxygen 

 tension in the blastocoel falls below that in the external medium. 



Early cleavage and early blastula stages of the asteroid Patiria show 

 a more strongly marked basipetal reduction gradient than the echinoids, 

 that is, the difference in rate of reduction at the two poles is greater. The 

 susceptibility gradient is also distinct in the early blastula and basipetal, 

 like the reduction gradient (Child, igi$a, 1916a). 



THE LATER BLASTULA 



In later blastula stages a change in the reduction picture occurs. A sec- 

 ond reduction gradient appears in the basal region, extends acropetally, 

 and partly obliterates the primary gradient. In the echinoids the center 

 of this secondary gradient system is the basal pole where the mesenchyme 

 cells, products of the micromeres, lie; but the adjoining entoderm reduces 

 almost as rapidly (Fig. 46). As immigration of the mesenchyme cells be- 

 gins, they become the most rapidly reducing cells of the whole individual, 

 and the prospective entoderm is only slightly less rapid (Fig. 46, B, E). 



Formation of primary mesenchyme preceding gastrulation does not 

 normally occur in asteroids, but a secondary gradient develops in the 

 basal region of the later Patiria blastula, though usually not as strongly 

 marked and not extending as far acropetally as in the echinoids; earlier 

 reduction on the presumably ventral side is also evident (Fig. 47). 



The earlier observations on differential susceptibility of the later blas- 

 tula stages in the sea urchin Arhacia punctulata and the starfish Asterias 

 forbesii agree with the data on dye reduction in that they show a decrease 

 in susceptibility from the apical pole basipetally and from one side, pre- 

 sumably ventral, according to evidence from gastrula stages; but they do 

 not show increase in susceptibility of mesenchyme and prospective ento- 

 derm (Child, 1915a, 1916a). More recent data on differential susceptibil- 

 ity to cyanide, lack of oxygen, and the dyes methylene blue and Janus 

 green do show this increase in Strongylocentrotiis; but the basal gradient is 

 more narrowly limited to the mid-basal region than is the dye-reduction 

 gradient, as comparison of Figure 48 with Figure 46 will show (see Appen- 

 dix IV, p. 739). The presumably ventrodorsal differential is evident in 

 Figure 48. In the asteroid basal increase in susceptibility does not appear 

 until later. 



