CLEAVAGE IN TOXOPNEUSTES VARIEGATUS. 147 



regard to the four arch-mesenchyme cells which Fleischman 

 found. Schmidt (1904) observed mesenchyme cells in the bias- 

 tula of Echinus microtubercidatus. They were present at the 

 seventeen-hour stage and continued to be formed until the 

 twenty-four hour stage. Boveri (1901) in his work on Strongylo- 

 centrotus Hindus, found in the unsegmented egg a portion at the 

 vegetal pole that was unpigmented. This region gave rise to 

 the micromeres, and they in turn to mesenchyme in the blastula 

 stage. The lack of pigment in these cells made their develop- 

 ment easy to follow. 



Tennent (1911) noted the presence of four mesenchyme cells 

 in the blastocoale of the Toxopneustes blastula about eight hours 

 after the fertilization of the egg and about an hour before the 

 beginning of gastrulation. An exception to the typical echinoid 

 condition is shown by Cidaris tribuloides (Tennent, 1914), in 

 which the formation of the mesenchyme is delayed and is given 

 off from the inner end of the archenteron in the gastrula. 



To our knowledge of mesenchyme formation in the Holothuroi- 

 dea the observations of Clark (1898) that in the blastula of 

 Synapta there is no evidence of mesenchyme cells, but that they 

 arise later, wandering in from the whole surface of the archen- 

 teron, and of Edwards (1909) that in Holothuria floridana the 

 mesenchyme begins by cell-proliferation at the vegetative pole 

 of the blastula, while at the same time gastrulation takes place, 

 confirm the earlier evidence of Korschelt, Selenka and Ludwig 

 to the effect that in the Holothuroidea the time of mesenchyme 

 formation varies. 



Bury (1888) and Seeliger (1892) from their study of the de- 

 velopment of Antedon rosacea agree that the formation of the 

 mesenchyme does not begin until after the beginning of the 

 processes of gastrulation. 



From these accounts it may be seen that, with the exception of 

 Boveri's work on Strongylocentrotus and MacBride's work on 

 Echinus esculentns, little attempt has been made to trace the 

 origin of the mesenchyme cells further back than to the approxi- 

 mate point in the larva from which they arise. If formed early 

 they come from the vegetal pole of the blastula, if late from the 

 archenteron. Theel makes the statement that " the mesenchyme 



