CLEAVAGE AND DEVELOPMENTAL PATTERN 593 



normal sea water, and how far return of cells to normal relations takes 

 place is uncertain. Driesch maintains, however, that normal development 

 may result, although blastomeres, except the micromeres, remain vari- 

 ously dislocated, and agrees essentially with Boveri (1901a, b) that the 

 vegetal region determines other parts. Since entodermization of prospec- 

 tive ectoderm, ectodermization of prospective entoderm and micromeres, 

 and induction of invagination from prospective ectoderm by implanted 

 micromeres have been shown to take place, normal development should 

 be possible with considerable dislocation of blastomeres. 



Cerehratulus eggs from which membranes have been removed by shak- 

 ing give rise in calcium-free sea water to a ring of eight cells, or a plate 

 of two rows of four each, or in some cases to eight cells in a single series 

 (Yatsu, 1910^). In all these the third cleavage, like first and second, is 

 vertical to the underlying surface. On return to normal sea water at this 

 stage, all eight cells may give rise to micromeres and rings; plates and 

 single rows of cells may develop into pilidia, some of which appear nor- 

 mal, while in others the apical organ is absent or doubled. Either the dis- 

 located blastomeres regain a more or less normal arrangement, as Yatsu 

 suggests, or there is partial redetermination of pattern by one or more 

 dominant regions. Cerehratulus embryos, kept in calcium-free water for 

 longer periods, form solid irregular cell masses with little visible differen- 

 tiation, except an apical flagellum and ciliated cells in some. Returned to 

 normal sea water, these masses differentiate ectoderm and gut but do 

 not attain full pilidium development. Some of Yatsu's figures suggest 

 differential inhibition of development, greater in ectoderm than in ento- 

 derm. 



ALTERATIONS OF CLEAVAGE BY DISPERMY AND POLYSPERMY 



Effects of dispermy and polyspermy have been most extensively stud- 

 ied in sea-urchin eggs. The first cleavage of a dispermic egg may be with 

 two separate spindles into two cells, or with monasters or various forms 

 of multipolar spindles into three or four cells simultaneously, and the 

 patterns of following cleavages differ from normal. Normal plutei develop 

 from only a small proportion of these eggs, and often development does 

 not continue beyond blastula or gastrula stages. Many of the forms re- 

 semble those resulting from differential inhibition by external agents. 

 The blastocoel is often more or less filled with free cells, as in differentially 

 inhibited forms; and cells may also be given off externally, also as in dif- 

 ferential inhibition (chap. vi). Cytological studies by Boveri showed that 



