20 Edmund B. Wilson. 



I have thus far been able to devote to the subject. The greatest 

 gap in my work thus far is the failure to trace the connected 

 history of the mesoblast, which can only be done by a complete 

 study of the cell-lineage. This presents considerable obstacles 

 owing to the difficulty of obtaining good total preparations at 

 every stage (the eggs and embryos stain diffusely in most dyes, and 

 the great abundance of deeply staining yolk in all the cells renders 

 it difficult to get clear pictures) , and my time was so taken up with 

 the study of the living material that I had not opportunity to 

 work out a really satisfactory method. For sectioning the best 

 results were given by sublimate-acetic, the sections being stained 

 with thionin, which gives a sharp nuclear stain without coloring 

 the yolk. The best total preparations were obtained by mount- 

 ing in balsam without staining. Apart from the technical diffi- 

 culties, the object is Itself difficult, in the earlier larval stages on 

 account of the difficulty of distinguishing between mesoblastic 

 and entoblastic elements in the crowded mesentoblast-mass, in the 

 later ones by reason of the complication introduced by the folding 

 of the mantle and the shell-gland. 



EXPERIMENTAL PART. 



The ease with which the eggs of Dentaluim may be operated 

 recalls the remark of Lacaze that "L'embryon du Dentale est un 

 de ces exemples f aits pour I'etude du developpement" ('57, p. 196). 

 For experimental purposes however it presents certain difficulties 

 that should carefully be borne in mind in considering the results 

 of the operations. First, there is a certain amount of variation, 

 not wide but still noticeable, in the size of the eggs and the re- 

 sulting larvae, and in the relative size of the polar lobe and of the 

 blastomeres during the cleavage-stages. Second, a certain pro- 

 portion of the entire eggs sooner or later develop abnormally, 

 which results in an Increasing mortality from day to day. Third, 

 and most important, the percentage of monstrous forms, and the 

 mortality. Is always very large in the development of egg-frag- 

 ments and of Isolated blastomeres. This Is undoubtedly due in 

 part to the abnormal conditions under which the larvae are placed 

 In the aquarium, In part to the shock of the operation, and in part 



