PHYLUM ANNELIDA 



287 



in norynal development. You will recall (p. 2U2) that isolated coelen- 

 terate embryo parts usually become whole organisms, whereas isolated 

 parts ot the ctenophore embryo become only portions of adults. The 

 annelid-mollusc trochophore is a classic example of the second type, in 

 which development is a mosaic. Each piece is able when isolated to 

 produce only those structures that it produces under normal conditions. 

 E. B. Wilson, a pioneer in expernnental embryology, separated the 

 cells of a cleaving mollusc egg in 1904, and found that each cell gave 

 rise to only a portion of a trochophore. In 1945 D. P. Costello did the 

 same with an annelid egg (Fig. 15.11, right). In his experiments Costello 

 separated the cells of the two-cell stage as soon as they formed, and 

 continued to separate cells as cleavage occurred until he had 16 cells in 

 16 separate dishes. Thus, none of the cells had any opportunity to in- 

 fluence any of the others. The 16 cells were then allowed to develop, 

 without further separation of cells. Sixteen groups of cells, four of each of 

 the varieties shown, resulted. Four dishes each had a cluster of small 

 cells some of which had cilia similar to those of the apical organ. An- 

 other four dishes each had a cluster of four large cells, three of which 

 had cilia like those of the prototroch. In the trochophore of the species 

 Costello studied the prototroch is formed by a circle of twelve large 

 cells, and just above the prototroch are four more large cells. Thus it 

 appears that the isolated cells formed exactly the number and kinds of 

 cells they form in the normal larva. Another four dishes each contained 

 a cluster of small cells which were identified as the progeny of the 

 second quartette. The four large cells of the sixteen cell stage each be- 

 came a single large cell with a cluster of small cells. In each case the 

 small cells were spread out over the surface of the large cell, suggesting 

 the only attempts at gastrulation found in the 16 isolates. From this 

 observation Costello concluded that the macromeres are necessary for 



First 

 ''quartette 



Second 

 cJuai'bctte. 



Third 

 c[ua.i'tetbe 



Mesendoderm. 



Figure 15.11. Development of the trochophore. The contribution of each tier of 

 four cells in the 16-cell stage (center) to the trochophore (left) is shown. The wavy 

 boundary between the ectoderm of the second and third quartettes is intended to show 

 interdigitation between these components and a degree of variability. \Vhen the cells of 

 the 16-cell stage are isolated, each produces a structure of the kind shown at the right 

 (four of each kind, sixteen in all). (Figures on the right are after Costello.) 



