548 MEMOiKS OF THE NATIONAL ACADEMY OF SCIENCES. 



PLATE XLIII. 

 (Stage VI.) 



FIGS. 123, 124. Completion of .scries of transverse sections of egg-nauplius. Cells marked Mes. 



probably represent emloderm in Fig. 121. The heart is being formed at about this 



time out of mesoblast cells at //, Fig. XLIII, and the endoderui forms a plate between 



it and the central yolk (V, Kig. 133). x295. 

 FIG. 125. Median longitudinal section of same stage. Compare with Fig. 106. The thoracico- 



abdominal fold is now distinctly directed forward, an i is overgrowing the sternal 



area between it and the mouth. The stomodaevim is a bent tube.. x295. 

 FIG. 126. Transverse section, cutting proctodaMiin. From an embryo of about the same age as 



that represented in Fig. 100. x295. 

 FIG. 127. Transverse section of embryo and entire egg on level with anus, showing wandering 



cells (Y. C., Y. C. 1 -' 2 ). x7i. 



REFERENCE LETTERS. 



J., anal invagiuation. 

 .Jfc., abdomen. 

 1. ) . \, altered food yolk. 

 '''', eggshell. 

 Ect., EI>., vetoileriu. 

 '/., gaiigliouic rudiiueut. 

 H., rudiment of heart. 

 IIij., intestiue. 

 f.h., labruin. 

 Mil., I'lnbryouii' mult. 

 .l/c., mesoderm. 

 Mo., mouth. 



I'll., ivijioii of i>roctoilii'iil invagiualiuu. 

 ., s 1 ., products of cell degeneration. 



s '. ' '., wandering cells, probably in early stages of degeneration. 

 >'. <>. <-.. rudiinrut of brain. 

 tt. ./., sternal area. 



)'., yolk. 



F. ''., 1 ".(''.'-', \vauderiug cells. 



)'. .S'. , yolk spherules. 



Vac., vacuole. 



