454 



W. DE MOKGAN AND THE LATE G. HAROLD DREW, 



are not present or are not recognizable, and have probably been dis- 

 solved away. Some irregular darkly staining masses suggestive of 

 endodermal cells are present, but they are somewhat indefinite. The 

 more distinct ectoderm cells are often arranged in whorls or rows, and 



ect. 



^X^^^ 



jiV'v^A^^_v> 







Fig. 7. x 260. — Section^through part of a restitution mass 6 clays old, showing indefinite 

 arrangement of cells. The ectodermal cells are fairly well differentiated ; but 

 the endodermal elements, though showing a tendency towards tubule formation, 

 are not^well defined. The perisarc is not shown. Ect., ectoderm cells. 



the external layer which secreted the perisarc is well defined. The im- 

 pression conveyed by examination of sections at this stage is that some 

 process of rearrangement has been initiated among the cells, but there 

 is little to show what may be expected to be the result of this rearrange- 

 ment. No mitoses w^ere observed. 



Seven days later development has proceeded much further, many cells 

 definitely of the endodermal type are present, and they contain numbers 

 of small granules in their cytoplasm. These cells are often arranged so 

 as to form distinct tubules, each having a definite lumen and closely re" 

 sembling in structure the coenosarcal part of the enteron of an individual 

 polyp. In other places the endodermal cells are arranged in rows, in 

 irregular masses, or singly, embedded among the ectodermal cells. Where 

 formed, the tubules are alw^ays in any one section cut transversely, longi- 

 tudinally, and at intermediate angles, hence they must be irregularly 

 coiled and crossed within the mass. At this stage the ectodermal cells 



