CHEMODIFFERENTIATION 57 



showing an overdevelopment of the organs concerned. An 

 isolated group of blastomeres without pole-plasm developed 

 into a defective embryo (Fig. 21a-c). Under abnormal cir- 

 cumstances the polar lobe may be distributed equally over the 

 first two blastomeres; this results in the formation of a double 

 monster, in the same way as in Tubifex. But if the two blast- 

 omeres of such eggs become completely separated, each will 

 develop into a complete embryo (Titlebaum, 1928). Wilson 

 (1929) studied the development of fragments of eggs which 

 had been halved before the beginning of cleavage. He found 

 that the pole-plasm was already present in the unfertilised egg. 

 At first, it is distributed homogeneously over the egg plasm; 

 later it assembles near the vegetative pole. This does not always 

 happen at the same time; in some species, it takes place in the 

 unfertilised egg, in others immediately after fertilisation, or 

 a short time before the first cleavage. 



Interesting results were also obtained by Wilson in his work 

 on the egg of the mollusc Dentdlium. In this species, a polar 

 lobe is formed three times, viz. at the first, second, and third 

 cleavages (Fig. 20). Dentalium has a so-called trochophore 

 larva, i.e. a larva characterized by a ring of cilia all round the 

 body. This ring separates the anterior pretrochal region from 

 the posterior posttrochal region. The animal side of the larva 

 bears a group of cilia, the so-called apical organ. Removal of 

 the polar lobe at the first cleavage results in a larva without 

 an apical organ, and with a reduced posttrochal region (Fig. 

 21d). Removal of the polar lobe during the second cleavage 

 produces larvae in which the posttrochal region is still reduced, 

 but which usually possess an apical organ (Fig. 21f). Evidently, 

 the factor responsible for the development of an apical organ 

 is located in the first, but not in the second polar lobe. There- 

 fore, in the interval between first and second cleavages, a shift 

 of the determining substances concerned must take place in 

 the egg protoplasm. This shows that these substances are "pre- 

 formed" but not "prelocalised" in the egg; they can change 

 their position during development. 



The development of the ascidian egg provides another good 

 example of the local accumulation of determining substances. 



