EXOGASTRULATION AND RELATED MODIFICATION-. 07 



marked by the "cell ro-ette" of Herbst, composed sometimes of 

 i-ol.ited cells and -onetimes of loose masses, as if the bla>tnla 

 wall had ,n inally bnr>t at its weakest point through di-teinion 

 of it- i , ; \ ity. Shortly the entoderm begins to evert, all stages of 

 ^astrulation an- present (Fig. 3), and soon the culture con- 

 -i-t- almost entirely of exogastrula? with only a few forms ap- 

 pro.K liin- normal endogastrulation. And here again one finds 

 the whole gamut of types with regard to the relative si/e of the 

 dermal and entodcrmal portions. The ectoderm shows all 

 d< ^rees of reduction down to complete obliteration in the large, 

 relatively clear, holentogastrulu', whose walls and content - 

 exhibit all the distinguishing features of the archentenm. Noth- 

 ing is more evident than that the entoderm grows here at the 

 ense of the ectoderm. 



At 33 36 hours or more those types in which both component - 

 are present may shoxv further differentiation: the archenteron 

 ^hows constrictions separating it into two, or its usual three 

 di\isions. This tri-partile gut is safely homologized with tore-, 

 mid-, and hind-gut, with the fore-gut of course, situated at the 

 most posterior free end of the embryo. The relative size of these 

 three divisions shows a most interesting series of variations 

 I ig. 4). The total gut is greatly increased in bulk over the 



ft C < 



Fit;. 4. Sck-rU'd series of types from saii<l-<l<>ll.tr i iiltun - <l, V i I.,IM-.I m Lit I. 



M/I5O to M/2OO. Showing, a-e, the wi> :--iIiii-ti"ii-i in th- m.il 



portion u'c/.i; ami tin- parallel ih pi< ipi>rt inn of tin- riiilncl.-i in n it-; 

 throe divisions: hind-gut (In; inid-giit (m); ami i 



normal and could not coin ei\ abl\- be em -lo-ed in the ravity of 

 the ectoderm; and in thi> inrn-a-e the (We-yin t.ike- the greatest 

 >hare, the mid -gut next, and the hind-^nt .ittai he<l to the ecto- 

 derm) the least. There is an ol>\ ion- and -i^nilicant parallel 

 in thi> proce to the relatixe increase of the etiloderm as a whole 

 at the expense of reduction of tin- ectoderm. 



