2 1 6 KINGSLE Y. [Vol. V 1 1 1 . 



middle line, are seen to have assumed a columnar character 

 and to be nearly free from yolk, while on either side they pass 

 into a tissue crowded with yolk spherules {ys) in which the cell 

 boundaries cannot be followed and in which the nuclei are 

 irregularly arranged. In Fig. 84 — a transverse section of a 

 slightly older embryo — the same conditions are shown upon a 

 smaller scale. On the upper left side of the intestine {mes.) the 

 cells have a well-marked epithelial character, while on either 

 hand they pass directly into the normal yolk cells of the earlier 

 condition. In a slightly older individual (Fig. 81) the whole 

 anterior end of the intestine is free from yolk, and its lining 

 cells (represented diagrammatically) have the character of a col- 

 umnar epithelium, while at the posterior end they pass directly 

 into the yolk cells which still fill the whole cavity in this region. 



This rearrangement of the epithelium is well advanced in 

 the intestine before it begins in the liver, and it advances more 

 rapidly in the central than in the peripheral parts of the latter. 

 It thus forms first the epithelium of the hepatic ducts (Figs. 

 72, ^6) and then the secretory epithelium. In the latter I have 

 failed to recognize an early differentiation of purely epithelial 

 and excretory cells such as has been described in some spiders. 



As will be seen, I regard the yolk in Limulus from the time 

 of its delamination as true entoderm. I fail to recognize, at 

 least here, the existence of " vitellophags " whose purpose is 

 merely the metabolization of the yolk and which then degen- 

 erate. I look upon the yolk cells from the beginning as 

 morphologically a true epithelium, the cells of which, being 

 gorged with yolk, are crowded from their proper position, thus 

 obliterating the lumen and obscuring their true nature. In 

 the later stages, when there is a rapid development of tissues, 

 there is a corresponding call upon the entodermal structures for 

 nourishment. Then it is that the yolk cells act temporarily as 

 " vitellophags " and, metabolizing the yolk, pass the products on 

 to the other tissues. It is only then that, the yolk being out of the 

 way, they are able to rearrange themselves as a true epithelium. 



While this view is in full accord with the observations of 

 most students of Arachnid development, it is at variance with 

 some of the commonly received ideas of Arthropod embryology. 



