592 PATTERNS AND PROBLEMS OF DEVELOPMENT 



age, the third cleavage is meridional, instead of equatorial, and a plate 

 of eight cells is formed. On removal of pressure at this stage, the eight 

 cells, or most of them, particularly those including the region about the 

 apical pole, give rise to micromeres by divisions vertical to those that 

 formed the plate.^^ Normal trochophores may develop from these forms 

 (Wilson); but, according to Morgan, all are at least slightly abnormal. 

 Alteration of cleavage pattern by pressure in the ascidian Ciona prac- 

 tically always results in abnormal development (Morgan, 1910a). Here 

 cleavage pattern is apparently more closely associated with developmen- 

 tal pattern. Only the mutual relations of the first three cleavage planes 

 of the ctenophore egg have thus far been altered by pressure. Unequal 

 divisions occur under pressure, as normally, but later development has 

 not been followed (Ziegler, 1898). In the egg of Ascaris little change in 

 cleavage pattern has resulted from pressure (Girgolaff, 191 1; Bonfig, 



1925)- 



Since it was discovered by Herbst (1900) that in calcium-free sea water 

 blastomeres of sea-urchin eggs do not flatten against each other but re- 

 main spherical and may become completely separated, this procedure has 

 been widely used as a means of obtaining isolated blastomeres or blasto- 

 mere groups and also in some cases for dislocation of blastomeres. Cal- 

 cium-free sea water alters condition of the ectoplasmic layer which holds 

 the blastomeres together. On return to normal sea water the change is 

 more or less completely reversed, and blastomeres again adhere normally. 

 Herbst observed that cell division continued in the calcium-free water and 

 that even isolated cells might become ciliated. Somewhat later Driesch 

 (1902a) found that dislocation of blastomeres, varying from one individual 

 to another, could be obtained by exposure to calcium-free water and that, 

 on return to normal, sea water development proceeded. When not dis- 

 turbed, sea-urchin eggs in calcium-free water tend to form a cell plate; 

 and apical or basal cells, normally in contact, may be widely separated. 

 Dislocation of the four basal cells of the eight-cell stage into two sepa- 

 rated pairs results in formation of two groups of micromeres; and, if these 

 remain separated after return to normal sea water, two invaginations and 

 two archentera may develop. These results are in line with those of Hor- 

 stadius on transplantation of micromeres (pp. 440-45). In spite of ex- 

 tensive dislocations of blastomeres, however, many of the more or less 

 platelike blastomere groups develop into normal plutei. There is consid- 

 erable change in cell positions as the mass becomes rounded on return to 



'^Nereis: E. B. Wilson, 1896; Morgan, 1910a. Crepiditla: Conklin, 1912. 



