438 PATTERNS AND PROBLEMS OF DEVELOPMENT 



suggest rather intense activation in the basal region; a developmental 

 activation certainly occurs. 



The first two divisions of the sea-urchin egg are meridional ; the third 

 is equatorial, apical and basal cells being approximately equal. In the 

 fourth cleavage the four apical cells divide meridionally, forming a ring 

 of eight cells (mesomeres), the four basal cells transversely and very un- 

 equally into the four micromeres at the basal pole and four large cells 

 (macromeres). Next the eight apical cells divide transversely and equally, 

 forming two rings of eight cells each, designated an^ and an2 by Hor- 

 stadius (Fig. 145, A). Somewhat later the four macromeres divide me- 

 ridionally, forming a ring of eight cells, then divide transversely, forming 



arif 

 an 2 



Fig. 145, .4, B. — Cleavage stages of sea urchin with Horstadius' designations of the four 

 rings of cells used in experiment; atii, auz, apical (animal) and subapical ring derived from the 

 four apical cells of eight-cell stage; vegi, vegz, rings derived from the four macromeres of sixteen- 

 cell stage which are basal (vegetal) except for the micromeres. 



two rings of eight cells each, veg^ and veg, (Horstadius), as indicated in 

 Figure 145, B. Earlier workers disagreed as regards the portion invagi- 

 nated as entoderm, but by means of vital staining of particular rings of 

 blastomeres it seems to be demonstrated that an^, aHi, and vegi all nor- 

 mally form ectoderm and that only the cells of veg, invaginate (von Ubisch, 

 1933; Horstadius, 1935, 1936a). 



With the aid of temporary exposure to calcium-free sea water Hor- 

 stadius has isolated blastomeres and groups of blastomeres as desired — 

 even single, two, three, or four micromeres — and, since the cells adhere 

 readily on contact, has been able to make various combinations of blasto- 

 meres or rings of blastomeres and to implant different numbers of micro- 

 meres in various relations to other cells. Staining of particular cells or 

 groups provides a means of identification, and local staining of isolated 

 parts serves to indicate axes, surfaces of separation, etc. In these experi- 



