1904.] NATURAL SCIENCES OF PHILADELPHIA. 393 



blast. The mesoblast arises in part from the fourth quartet cell 

 of D quadrant. The remaining fourth quartet cells and all the 

 macromeres are entoblastic, as is also the case with a small portion 

 of 4d. 



The first quartet of ectomeres give rise to the trochoblasts and ecto- 

 blastic cross. To the latter structure are added as "tips" the upper 

 cells of the second quartet m all quadrants. The cross is radially- 

 spiral in symmetry, and does not increase in breadth by transverse 

 sphtting of its arms until a comparatively late period. Cells from the 

 fu-st quartet form the head vesicle, cerebral ganglia and eyes, and a 

 portion of the first velar row. 



The second quartet has a similar cleavage history in all four quad- 

 rants until a stage of about 150 cells. In later development the 

 elements of this quartet in D (posterior) quadrant show great 

 increase in size and divisional activity, initiating the posterior point 

 of growth, with resulting bending of the embryonic axis. Cells from 

 this area form the shell-gland and median portion of the foot. A 

 large number of second quartet cells from the anterior and lateral 

 groups aid in the formation of the velum. The more ventral elements 

 of B quadrant help to close the blastopore. 



In the third quartet bilateral cleavages first appear in the posterior 

 quadrants (cells 3c^ and 3d^). Secondary mesoblasts arise from the 

 anterior quadrant groups of this quartet (cells 3a^"S 3a"" and 3b^"^ 

 3b^^"). The large anal excretory cell (3c"") and its associated cells 

 are derived from C quadrant of this quartet. Third quartet cells 

 surround the blastopore as it closes, with the exception of a small 

 anterior portion; much of the stomodseum and the lateral portions 

 of the foot come from third quartet elements. 



The mesoblast of Fiona is derived from two sources, ento- mesoblast 

 from 4d and ecto-mesoblast from the third quartet in A and B quad- 

 rants. The greater amount comes from 4d and forms teloblastic 

 bands in the posterior region of the gastrula. The secondary mesoblast 

 (ecto-mesoblast) is largely "larval" in fate, since much of it goes to 

 form the muscles of the velum. From the history of 4d it appears 

 that this cell contains both mesoblastic and entoblastic derivatives, 

 the latter taking part in the formation of the intestine. 



As is the case with many Opisthobranchs, the gastrula is sharply 

 pointed anteriorly, the apical point at first lying at the end of the 

 anterior arm of the cross. 



The blastopore at the time of closure is surrounded by third 

 quartet cells, except at its anterior edge, where second quartet cells are 



