320 



Embryogenesis: Progressive Differentiation 



CD cell when the first cleavage is completed 

 and the D cell when the second cleavage 

 has terminated. The trochophore larva of 

 Dentalium is illustrated in Figure 120G. If, 

 in Dentalium (Wilson, '04a), the AB blasto- 

 mere or the A, B, or C blastomeres, all of 

 which lack the substance of the polar lobes, 

 are isolated, they develop into larvae lacking 

 an apical tuft and a post-trochal region (Fig. 

 120H). The same type of defective larva 

 develops if the first polar lobe is extirpated 

 while the first cleavage furrow is forming. 

 Thus it appears that the formation of apical 

 tuft and post-trochal region is dependent 

 upon that part of the cytoplasm contained 

 in the first polar lobe. If the first polar lobe 

 is not removed, but instead the second one 

 is extirpated, the resulting larva develops an 

 apical tuft, but still lacks the post-trochal 

 region (Fig. 120/). Thus it appears that the 

 portion of the cytoplasm essential for apical 

 tuft formation is not present in the second 

 polar lobe, although the portion of the cyto- 

 plasm essential for formation of the post- 

 trochal region is contained therein. At the 

 third cleavage the first quartette of micro- 

 meres forms, and when the cells of this 

 quartette {la-id) are isolated, only cell 

 id forms an apical tuft (none of the micro- 

 meres forms a post-trochal region). Thus it 

 appears that the portion of the cytoplasm 

 essential for apical ttift formation is progres- 

 sively translocated from the first polar lobe 

 into the CD cell and finally into the id 

 blastomere. The latter cell alone is then able 

 to develop into a larva with an apical tuft 

 because a special portion of the heterogene- 

 ous cytoplasm has been segregated into it. 

 Similarly the iD cell receives the portion of 

 the cytoplasm contained in the second polar 

 lobe, and it alone of the macromeres of the 

 8-cell stage has the capacity to form the nost- 

 trochal region. Strikingly similar results have 

 been obtained with pe;gs of the annelid 

 Sabellaria by Hatt ('32) and Novikoff ('38), 

 although there are differences in detail. 



Experiments on Esejs Possessing Pole Plasms. 

 The eggs of Olios-ochaeta and Hirudinea are 

 characterized by the accumulation of an area 

 of distinctive protoplasm at the animal pole 

 and another one at the vee:etal pole iust after 

 the second maturation division; such local- 

 ized protoplasmic areas are termed pole 

 plasms. In Tubifex (Penners, '22) these two 

 pole plasms are progressivelv restricted dur- 

 inff cleavage to the CD cell, then to the D 

 cell where they fuse, then to the iD cell. 

 About half this pole plasm then enters the 

 2d cell (first somatoblast— see Fig. 120B) of 



the second quartette and finally enters one 

 of its derivatives (2cZi^i), which then consists 

 almost exclusively of pole plasm. This cell 

 divides into Tl (to the left of the dorsal 

 midline) and Tr (to the right), and from 

 these two cells all ectodermal structures are 

 derived (ventral nerve cord, cerebral gang- 

 lion, circumesophageal commissures, circular 

 muscle, epidermis, and probably the setal 

 sacs). The rest of the pole plasm (Penners, 

 '24a), after passing through cells 2D and 3D, 

 finally enters cell 4d (second somatoblast — 

 see Fig. 120S) which divides into Myl (to the 

 left of the dorsal midline) and Myr (to the 

 right). From these two cells all mesodermal 

 structures are derived (the somatic meso- 

 derm and the longitudinal musculature de- 

 rived from it, the splanchnic mesoderm, the 

 septa and the nephridia). The possibility 

 exists that the portion of the pole plasm 

 entering cell 2^^^^ specifies the derivatives of 

 that cell to become ectodermal structvires, 

 whereas the portion of the pole plasm enter- 

 ing cell 4d specifies the derivatives of that 

 cell to become mesodermal structures. 



Blastomeres of Tubifex eggs are very sensi- 

 tive to radiation by ultraviolet light; indi- 

 vidual radiated blastomeres are cast out from 

 the developing system by non-radiated cells 

 (Penners, '26). As stated above, half of the 

 pole plasm appears to be segregated progres- 

 sively into cells 2D, 3D, 4d and finally Mvl 

 and Myr; mesodermal structures fail to form 

 if any of these blastomeres are destroyed. The 

 other blastomeres develop quite normallv 

 but parts are frequently displaced from their 

 normal positions relative to one another and 

 metamerism is lacking. Penners ('38) con- 

 cluded that the pole plasm passing through 

 the blastomeres listed above specified Mvl 

 and Myr for mesodermal differentiation and 

 further that the mesoderm must exert some 

 organizing influence upon the ectoderm and 

 entoderm. Similarly, half of the pole plasm 

 appears to be segregated progressively into 

 blastomeres, 2d, 2d^, 2d'^^ and 2cZ"i; in early 

 experiments (Penners, '26) ectodermal struc- 

 tures failed to form if any of these blasto- 

 meres were destroyed. The rest of the blasto- 

 meres developed quite normallv and Penners 

 concluded that the pole plasm passing 

 throuffh these blastomeres specified 2^i^i for 

 ectodermal differentiation. However, Pen- 

 ners ('37) discovered that if he kept these 

 embryos alive as long as possible, consider- 

 able regulation occurred in the absence of 

 derivatives of 2rfm, with all parts normallv 

 forming: from this cell now arising from 

 derivatives of Myl and Myr. Thus the meso- 



