r.H)7.] NATURAL SCIENCES OF PHILADELPHIA. 541 



upper ami lower eii<li»(leriii have separated off. ^\'ith regard to the 

 inidtlle endoderni cells he says, ]). 35G: "Die in ihnen enthaltenen 

 grobeii Dotterkonier scheinen mitcinaiider zu verschmelzen so dass 

 tlie in Frage stehenden Zellen das Aussehen von beinahe honiogenen, 

 stark lichtljrechender Fettkugeln l^ekamen. Ich habe in diesen 

 Kiigeln bei ihrem zerfall nie Anij)hiastcr sieh bilden sehen. obschon 

 ieli aiifnierksani danach gesucht habe." 



The breaking up of tliese yoHc cells is very irregular. The wall of the 

 alimentary caiuU, according to Lang, is formed by the cells of the 

 upper and lower entloderm. 'J'hese increase by division and extend 

 over the yolk spheres. Finally they unite to form a definite layer in 

 which the cell l)oundaries cannot be distinguished. The endoderm cells 

 have a more or less amoeboid character and send out protoplasmic 

 processes over the numerous 3'olk spheres. According to him the 

 middle endoderm (fourth quartet) contains onl}^ yolk granules and 

 does not take part directly in the formation of any organ. 



Throughout all these accounts one or two phenomena are constant. 

 e.g., the yolk breaks up into a large number of spherules which are later 

 absorbed by the endoderm cells. Witii regard to the development 

 of the canal itself there is some variation. In all the accounts at least 

 a portion of the canal is derived from the lower endoderm (macro- 

 meres). In some cases (Stylochus and Discoccelis) upper endoderm 

 cells are formed from the large j'olk cells. 



The account which I have to offer of the development of the ali- 

 mentary canal in Planocera inquUina differs from any of the above in 

 several particulars. At the time when the mesoblasts, Ad-'^- and 

 4d^---^, are preparing to divide, the two large entoblasts, Ad}'^ and Ad^'\ 

 are dividing (fig. 33). By this division two large cells, Ad'^'^- and Ad^'^-, 

 are budded into the interior of the embiyo. Soon after this these cells 

 divide again. At a considerably later stage the two cells Ad^-^-^ and 

 Ad^"^-^, -^vhich remained on the surface after the last division, divide 

 again, budding two more large cells into the lower ])art of the embryo. 

 At this time or shortly afterwards the ectoderm has covered this 

 region, and all six cells and their descendants originally derived from 

 the two primitive entoblasts, Ad^-^ and 4rf'-^, are on the interior of 

 the egg. 



Text fig. 3 shows in optical section an egg of a considerabl}' later 

 stage, in which a number of cells are lying just above the vegetative 

 pole. These cells, of which there are several more in the egg, all came 

 from the primitive entoblasts, 4f/'-^ and 4c/'-. By examining a large 

 nunib(M- of eggs it is found that these cells are in very active division 



