ox 'J'HE DEVELOPMENT OF LTMULUS LONGISPTXA. 89 



plate. The lateriil eyes;, however, gradnnlly travel fro7n the thickened 

 margin of tlie ventral p'ate exteriorly and posteriorly, until they are 

 f(innd in the d(^r>!nl surface of the embr3^o, behind the lateral hump in 

 the level of the f»MU-th tlK^racie appendaire. The invan-ination of the 

 median eye-; Ijecome.-^ o-radiially deep, and while tlie iiKjuth oï the 

 invagination dc.e^ not shift its ])lace, with respect to t1ie hrain, the in- 

 vaginated tnhe grows anteriorly arid dorsal ly, keeping its blind end al- 

 ways in contact with tlie epil)la.st. Its deNclopment soon surpasses that 

 of the thiekevied margin of the ventral plate, and at last itsti]) is found 

 a.t tlie dorsal surf ice <^f the emlrryo. Tn the trilobite-stage (fio-, 17) 

 the e{)iblast is tliickened in a line above the eves (tig. 64). 



I pr<~)pose to interpret the above stated facts as follows : The 

 epil)last, covering the body of LimnJm may be distinguished into the 

 dorsum tmd ventrum, and the line in whicli thev meet is clearlv 

 marked out as the external margin of the pleurae in tlie :djdomen, and 

 as the ftcial suture (the outer inargiii of the fixed cheeks) in the 

 cephalotliorax. The ventrum is the epiblast wliich develops from 

 tlie ii])[)er layer of cells of tlie blastodermie thickening, ami the dorsum 

 is the descendant of the epiblastic cells covering a.ll the surface of the 

 ego- exeept the blastodi'rmic thickening. Thus the free-cheeks whicii 

 are usually looked on as a {)art of the dorsum are in mv opinion, a ])art 

 of tlie veiitrum. Tf we acce])t the view that the origiiial blast<i]^(^»re 

 is in the ventral median line of the Artln'o]ioda and tlu- stomoda^um 

 arid the proctoda^um are its two ends, and that the segments of the 

 Arthropoda are homologous with the compartments of the Cu'lenterata, 

 theii tlie ventrum as defined aliove. corres])onds to the subinubrella of 

 the medusae, all the important organs being produced ii'(-iH it a?id 

 the dorsum to the exumbrella. 'i'he margin of the umbrella, i.i\, tlie 

 boundary line between the two, has all the sense-oi-gans on it. 



At the time when the eyes are first formed, the ventrum and the 



