1904.] NATURAL SCIENCES OF PHILADELPHIA. 721 



that ectomesoblast, if such exists, is very small in amount, and that 

 the greater part, if not all, of the mesenchyme of the adult as well as 

 the ovaries are formed from 4d. 



VIII. — History of the Fourth and Fifth Quartettes. 

 (1) The Entomeres. 



The entoderm, at the time 4d is formed (fig. 17), consists of 2A, 2B. 

 IC and 3D. The origin of the third quartette and the primary meso- 

 blast cell 4d have already been described. The other three members 

 of the fourth quartette arise by an equal and leiotropic cleavage. 

 Figs. 23 and 24 show the origin of 4c, fig. 29 that of 4a, while figs. 33 

 and 41 show 4b already formed. At the 72-cell stage 4D has divided 

 dexiotropically into 5D and 5d (fig. 33), the latter product being the 

 smaller. The inequality of this division compensates, so to speak, for 

 the inequality of the division preceding, since at the completion of 

 the seventh cleavage the original macromeres 5A, 5B, 5C and 5D are 

 alike in size. After the division of 4D, 4C, 4A and 4B divide equally 

 and dexiotropically to form the fifth quartette (figs. 51 and 52). The 

 original macromeres now form a cross, the arms of which lie radially. 

 Since up to the last division the macromeres lay interradially, it is 

 evident that a rotation of the macromeres through an arc of 45 degrees 

 has been brought about. This rotation has taken place in an anti- 

 clockwise direction, viewing the ovum from the animal pole, as can be 

 seen by a comparison of figs. 50 and 52. This movement is due to the 

 reduction in size of the macromeres and their superficial position at 

 the seventh cleavage. During this cleavage the members of the fifth 

 quartette are, on the other hand, held in place by their greater surface 

 contact with the surrounding cells of the egg, so that the macromeres 

 5A, etc., are rotated, as were the members of the first quartette at the 

 time of their origin, A similar rotation apparently occurs also in 

 Amphitrite, Clymenella (Mead, 1897) and Arenicola (Child, 1900). 



After the division of the fourth quartette the entomeres cease divid- 

 ing and enter a resting stage which continues until after the closure of 

 the blastopore. They now form a thick, roughly ovoid mass of cells, 

 fourteen in number. At the centre of this mass on the ventral side 

 (fig. 52) lie the four macromeres, 5D posterior, 5B anterior, while 5C 

 and 5A lie laterally. Alternating with the macromeres are the four 

 members of the fifth quartette, while at the outer ends of the cross 

 formed by the macromeres lie the three pairs of entomeres belonging 

 to the fourth quartette. This relation is very similar to that described 



