CLEAVAGE AND DEVELOPMENTAL PATTERN 



557 



unknown inhibiting factor which prevents normal union of germ bands 

 anteriorly. It may be suggested further that absence of setae and ne- 

 phridia on the apposed sides may result not from lack of capacity to re- 

 constitute these organs but from mutual inhibition of the two parts, as in 

 many cases of postembryonic rcconstitution. The other form of partial 

 twinning is a cruciate duplication, resulting from duphcation of the germ 

 bands and union of one band of each pair to form the two anterior ends 

 (Fig. 176). This type of duplication has also been produced experimen- 

 tally by subjecting eggs of Tubifcx to high 

 temperature together with low oxygen 

 content of water (Penners, 1924/'). Under 

 these conditions the first cleavage is equal 

 instead of unequal, as normally; and in 

 later cleavages two 2d- and two 4d-celh, 

 instead of one, appear. Equal division of 

 the cell CD at the second cleavage gives 

 the same result. Each of the 2d-ce\h 

 gives rise to a pair of ectodermal germ 

 bands, and each of the 4d-ce\h to a pair 

 of mesodermal bands. According to Pen- 

 ners, duplication of the somatoblasts 

 results from equal division of the polar 

 plasms (p. 550) by the equal first cleav- 

 age. 



Development of certain blastomeres or blastomere groups of Tubifex, 

 after killing others by localized ultra-violet radiation, high temperature, 

 or strong shaking, has also been followed (Penners, 1926). The dead cells 

 soon separate from the living. Further cleavage patterns of isolated CD- 

 or Z)-cells containing the polar plasms is not altered; they give rise to 

 the two somatoblasts, these to teloblasts and germ bands, and normal 

 embryos result. The germ band of one side can develop and differentiate 

 when that of the other side is killed. When the polar plasms are killed, 

 germ bands do not develop, and the embryo consists of a compact mass 

 of entoderm with an ectodermal cap of variable size. Ectodermal germ 

 bands do not develop when the first somatoblast is removed, but meso- 

 dermal bands develop and cUfferentiate. Killing of the cells 2D or jZ), 

 which are in the mesoderm line, or of the mesoblast 4d or its two daughter 

 cells, results in absence of mesoderm. On the other hand, normally pro- 

 portioned embryos develop when only part of the entoderm cells are pres- 



FiG. 176. — Posterior parts of ecto- 

 dermal germ bands in cruciate duplica- 

 tion of Tubifex (after Penners, 1924a). 



