442 BULLETIN 82, UNITED STATES NATIONAL, MUSEUM. 



and through the intermediary of globular cells. From the anterior end of this 

 sack mesenchyme cells continue to arit-e. 



When the mesenchyme cells are abundant many of them, through mutual 

 pressure, appear polyhedral. The difference in the chromatin content of the 

 nucleus in the resting stage and at various steps during mitosis is especially 

 striking. 



The relative paucity of chromatin in the nuclei in the resting stage lying in 

 a plasma alwaj's rich in yolk renders them very difficult to see, and this is possibly 

 the reason for the numerous figures of mesenchyme cells without nuclei which 

 have been published. 



In the entoderm cells also a change in the amount of chromatin in the nuclei 

 can be detected at different times, though the differences are less considerable 

 than in the mesenchyme cells. In the ectoderm cells, on the contrary, the amount 

 of chromatin appears to be constant at all times. 



In the following stage the posterior coalomic sack becomes more and more 

 elongated laterally, somewhat constricted centrally, and broadened at the two ends. 



The anterior section of the primitive gut has continued its growth about 

 the posterior section, which, with its recurved ends, it now incloses. 



In almost all of the embryos the ectoderm of the ventral side has now become 

 somewhat thicker than that of the dorsal, though frequently this first occurs some- 

 what later. This broad zone of higher ectodermal cells on the ventral side 

 indicates the position of the subsequent vestibular invagination. 



Histologically the conditions at this stage are essentially the same as in the 

 preceding. 



Slightly more developed embryos show the ectoderm becoming locally differ- 

 entiated, so that in longitudinal sections there can be distinguished four trans- 

 verse bands and a terminal ring in which the nuclei are more lightly colored 

 and lie more closely together than elsewhere. At this time these zones are broader 

 than the intermediate zones (though later the reverse is true) and are somewhat 

 irregular, of varying breadth and in places only slightly differentiated from the 

 neighboring region. The sharp differentiation of the ciliated bands from the 

 remainder of the ectoderm is first noticeable at about the sixtieth hour. 



The ccelomic sack has now become more constricted centrally, while the two 

 ends have become swollen anteriorly, so that two rather extensive lateral pockets 

 are formed which are connected by a minute canal. Since their growth has 

 been especially dorsalward they appear here much less widely separated than 

 ventrally. In a median section the canal connecting them shows a thick wall 

 with a very small lumen. Histologically the structure is essentially the same 

 as in the preceding stage. 



The two median processes of the anterior sack, which lie dorsally and ven- 

 trally, above and below the canal uniting the ccelomic pockets, have curved them- 

 selves around this last until their blind ends lie behind it. 



The more extensive central portion of the anterior sack lies closely appressed 

 to the canal imiting the two ccelomic sacks. Its walls are everywhere composed 



