EMBRYOLOGY OF THALASSEMA MELLITA 231 



V. Rudimentary Cells. 



Perhaps the most interesting feature in the cleavage of TJial- 

 assema is the seeming prodigaHty in the formation of cells that 

 degenerate and are ultimately completely absorbed by the ento- 

 blast. It is difficult to explain their existence save under the 

 assumption that they have lost their original functional signifi- 

 cance and only persist as a survival of an ancestral type of 

 cleavage. It is interesting to note that some of these cells, 

 when first formed, differ from their neighbors only in their 

 much smaller size and not in any special cytoplasmic or nuclear 

 feature. 



The following eleven cells are budded off early enough in 

 the cleavage to admit of an exact determination of their cell- 

 lineage and fate. From each of the posterior arms of the cross 

 arises one cell (i<^i. 1.2. 2. d 1^1.1.2.2.1)- (See PL I, Fig. 7 and 

 Text-Fig. 10, A). They are the same in origin with the ceils 

 which in Nereis (" nephroblasts "), Amp Jiitrite and Capitella 

 form glands. In Podarke, however, they are rudimentary and 

 have been tentatively described by Treadwell ('01) as suffering 

 a like fate to those in Thalasseuia. The first dexiotropic divi- 

 sions of the intergirdle cells of the a, b and c quadrants (PL I, 

 Fig. 4), as has already been indicated, give rise to three more rudi- 

 mentary cells (1^1.2.1.2. i*^!. 2. 1. 2' 1*^1. 2. 1.2)- Ii^ A)}ipJiitrite , too, 

 the cell, 1^1. 2.1. 2> is entirely rudimentary and the cells, 1^1.2.1.2 

 and i<^i. 2. i.2> very small. In Arenicola these same cells are 

 also very small, with dark staining nuclei. Child ('00) was 

 not able to determine their fate, but thinks it possible that they 

 become mucous glands. Another rudimentary cell in Thal- 

 assenia arises in the d intergirdle region (id^ o. 2. i> PI- H. Fig. 14). 

 An exactly similar cell in origin and appearance is figured by 

 Meade ('97) in Amphitrite. The corresponding cell in the b 

 intergirdle region (1^1 2. 2.1) is also rudimentary in Thalassema 

 (PL I, Fig. 6). The second quartet contributes the following 

 rudimentary cells, 2a.^_^, 2/a_, ^ , 2^:2.1 and ^1.1.2.1.1 (PL II, Figs. 

 15, 18, 20 and 21). In addition to these there are several 

 found during the later development, making a conservative esti- 



