The Development of Isehnochitou. 627 



pole. Iq ^ a and 4 c on the other hand the spindles are nearly hori- 

 zontal, and are directed as in Fig. 40 or slightly to the left. The 

 position taken by the daughter cells has not been observed. 



2. The Fifth Quartette. 



Invagination has advanced to a considerable degree and the 

 macromeres have pressed into the segmentation cavity and are in 

 contact with the ectoderm of the velar field before the spindles arise 

 which produce the fifth quartette. The macroraere D shows no sign 

 of division but the three remaining cells divide simultaneously, the 

 spindles being all directed toward a common point located about the 

 centre of the velar field. A considerable shifting occurs among the 

 macromeres and the newly formed fifth quartette which renders it 

 impossible to accurately trace their history into later stages. 



Macromere D divides in the stage represented by Figs. 39, 40. 



XI. The Free-Swimming Larva. 



1. Habits. 



Il has invariably happened that no matter how carefully taken 

 care of, the eggs of IscJinochiton will not segment normally after 

 being kept two or at least three days in the aquarium. It is therefore 

 not possible to take an egg through from the early stages until the 

 embryo leaves the egg membrane, if it be kept in the laboratory, but 

 if the egg strings soon after they are laid are carefully fastened 

 among the corallines in some comparatively quiet tide pool they will 

 then develop normally. 



The length of time from egg laying until the embryo becomes 

 free is seven days, and if one brings in the jelly masses on the seventh 

 morning after their deposition the larvae will be seen leaving the egg 

 envelopes in gradually increasing numbers until about noon, and in 

 diminishing numbers until evening. 



In the period comprised between the commencement and completion 

 of the shifting of the blastopore the embryo has a somewhat elongated 

 form but in later stages an increase in size takes place, whereby the 

 chorion becomes almost completely filled and the larva becomes more 

 spherical as a result. Immediately preceding the free swimming stage 

 spasmodic contractions and elongations of the trunk region are noticed 

 and it is possible that in this way the membranes become ruptured 

 and the larvae escape. These latter are heliotropic to the extent that 



