176 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



these embryos were killed in October. In most individuals of this age 

 there is no sign of ectodermic invagination in the region of the endsac. 

 The endsac (Plate 2, Fig. 25, sac. trm.) is oval and contains in different 

 individuals from 18 to 26 nuclei, which are often only partially separated 

 by cell walls, these being evident in the peripheral portions of the organ 

 only. The central area of the endsac is a granular mass with one or 

 more large irregular vacuoles, or several small ones, while the nuclei 

 lie peripherally in compartments between the incomplete cell walls. In 

 some cases the cell walls are completed, and thus a definite boundary to 

 the lumen is formed. This is the condition of the endsac when invagi- 

 nation of the ectoderm begins, but in some individuals, even after in- 

 vagination is well started, the cell walls of the endsac are incomplete and 

 its interior is still a granular mass with several vacuoles (Plate 3, Fig. 



26, sac. trm.^ vac). 



In some precocious individuals of this batch of embryos I find evi- 

 dence of the beginning of the invagination, — a slight depression of ecto- 

 derm cells. In Figure 25 (Plate 2) at the median ventral border of the 

 endsac appears a thickening of the ectoderm (i^vag. ec'drm.). This rep- 

 resents the beginning of the ectodermic ingrowth. 



In sections of the stages succeeding this it is seen that the ectodermic 

 growth extends proximad and dorsad.^ The axis of this line of growth 

 is not parallel with the antero-posterior axis of the body of the embryo, 

 but in passing proximad trends slightly laterad, so that true parasagittal 

 sections of the embryo cut the invagination obliquely. The ingrowth 

 consists of a plug, or perhaps better, a sheet of cells, the width of the 

 sheet being however small. This sheet grows up around the end- 

 sac and lies against its distal and dorsal faces (Plate 3, Figs. 26, 



27, i'vag. ec'drm.\ 



From all the evidence afforded by my sections I believe that this sheet 

 of cells arises by a process of growth which is most active at its free 

 (deep) end. The conclusive evidence must rest upon signs of mitosis in 

 numerous series of stages covering the period of this growth. I have not 

 sufficient material at command to settle the point definitely. 



However this may be, this ectodermic ingrowth occupies a narrow 

 space between the distal and dorsal faces of the endsac and the wall of 



^ In reforrinp: to the embryonic organ I must use designations of direction which 

 seem contradictory. For that face of tlie second antenna wliich lies nearest the 

 maiuhble and is now tlie dorsal face is ultimately the ventral, and must be so des- 

 ignated. It assumes its normal position at the time of hatching by revolving about 

 its base as a centre through an arc of about 135° in a parasagittal plane. 



