EMBRYOLOGY OF THALASSEMA MELLITA 181 



later cleavages has been omitted, as they are not of general in- 

 terest. They may, however, be followed in the figures (PI. I, 

 Figs. 5, 6, 7). The rosette cells divide once certainly, leio- 

 tropically in the a, b and c quadrants and bilaterally in the d 

 (PI. I, Figs. 6 and 7). Some of them have been observed to 

 divide again. All of these divisions take place after the cells have 

 developed cilia. From the rosette cells, I think, is developed 

 the entire apical plate. 



b. Second Quartet. — The second quartet claims our interest 

 from the fact that the posterior member (2<a?) has been found to 

 produce in a number of annelids and molluscs, almost the entire 

 post-trochal ectoderm and consequently that of the trunk of the 

 adult. In the unequal types of cleavage this protoblast is often 

 the largest cell in the ^^^ [Nereis^ Amphitrite, Arenicola, etc.). 

 Its origin and subsequent history were first accurately deter- 

 mined by Wilson ('92) in Nereis^ who designated it by the letter 

 X. Treadwell ('01) has recently discovered that in an annelid 

 with " equal " cleavage (^Podarke), although 2d is no larger 

 than the other three members of the quartet, yet it divides more 

 rapidly and its progeny increase in number and size to such an 

 extent as to give rise to a considerable part of the post-trochal 

 ectoderm and to the '* growing point." As will be seen later 

 the same is still more markedly the case in Thalassenia. The 

 cell may accordingly be said to be homoblastic and, to a large 

 extent, equivalent in both types of cleavage . I will first describe 

 its divisions, following Treadwell's nomenclature. 



X-G7'oiip. — As stated above, at the 64-cell stage, the sinis- 

 tral cell in the d quadrant of the second quartet divides off a 

 small cell downward and over the protoblast of the coelomeso- 

 blast (4<^) (PI. I, Fig. 11). The dextral cell next divides 

 somewhat unequally (PL I, Fig. 1 2). The upper smaller prod- 

 uct of this division is X^ and the lower X^. There follows a 

 leiotropic division of X^^ (PL II, Fig. 17). The upper and 

 somewhat smaller cell corresponds to the entirely degenerate 

 cells divided off earlier in the other quadrants (2^2.i> etc.) 

 ^1.1.2 very soon divides again and in the same direction (PL II, 

 Figs. 17 and 18). ^1. 1. 1 has increased, in the meantime, con- 



