EMBRYOLOGY OF THALASSEMA MELLITA 16^ 



has no difficulty in distinguishing the sexes by the difference in color 

 of the sexual products which show through the semi-transparent body- 

 wall. The eggs were preserved at intervals of 15 minutes during the 

 early cleavage stages and then every half hour until the trochophore 

 stage was reached. The most satisfactory killing agent proved to be 

 picro-acetic (with i to 2 per cent, acetic). Kleinenberg's picro- 

 sulphuric (dilute) was also of value, but Flemming's fluid was quite 

 useless, except for the preservation of cilia, even for sections. After 

 considerable experimentation, the best stain for total mounts was 

 found to be that first described by Conklin ('97), /. e., a weak solu- 

 tion of Delafield's haematoxylin slightly acidulated by a few drops 

 of Kleinenberg's picro-sulphuric solution. In general, the method 

 given by Child ('00) was followed in staining and mounting the 

 eggs. All the essential points determined in optical sections have 

 been confirmed by actual sections. These latter were stained with 

 iron hematoxylin with a Bordeaux or Congo red counter stain. The 

 figures are all from camera drawings. 



I. Cell Lineage to Gastrulation. 



General Sketch. — The following is the first description of the 

 cleavage in the Armata, considered from the cell-lineage stand- 

 ponit, and, as is naturally to be expected, it conforms closely to 

 the determinate ^ type that has been described by Wilson ('92), 

 Mead ('97), Child ('00), Treadwell ('01) and others to be the 

 case in a number of other annelids. The cleavage, however, is 

 equal and so closely resembles that oi Podarke (Treadwell, '01) 

 that the latter may be taken as a basis of comparison. Of other 

 representatives of this type the best known are Lepidonotiis 

 (Mead, '97), Hydroides (Wilson, '91), Eupomatns (Hatschek, 

 '85), Salnlla and Poniatoceros (von Drasche, '84). 



In accordance with the general rule, the first quartet of mi- 

 cromeres is formed at the third cleavage by a right-hand spiral 

 (dexiotropic) division of the four subequal " macromeres." 

 Then follow a second quartet by a left-handed (leiotropic) spiral 

 and a third quartet by a right-handed spiral cleavage. These 

 three quartets give rise to all of the ectoblast and also to all the 

 ectomesoblast. The fourth quartet arises leiotropically and the 



1 Spengel's ('79) figures indicate that the same is the case in Bonellia. 



