175 



rows of cells as the permanent nephridia in Clepsine, Nusbaiim's state- 

 ment that the nephridia arise in connection with the somatopleura, 

 is in plain contradiction with my observations. 



The permanent Entoderm. 



The epithelium of the whole alimentary tract, excluding the sto- 

 modaeum (pharynx) and proctodœum, which are derived from the 

 epidermal layer, arises from free nuclei belonging to the three large 

 blastomeres (a, b and c in my figures). The cells which form the œso- 

 phagus are the first in order of development, making their appearance 

 just beneath the stomodœal thickening, in the very earliest stage of the 

 germ-bands. From the mass of cells formed at this point arise not only 

 the oesophageal epithelium, but also the salivary glands. The method 

 employed gives preparations in which all the embryonic tissues of the 

 head and anterior portion of the trunk (epidermis, larval gland, salivary 

 glands, nerve-cells, muscle-cells, and oesophageal epithelium) are di- 

 stinguishable. 



The cells destined to form the epithelial lining of the stomach arise 

 later than those of the œsophagus. They appear first as distinct cells, 

 on the ventral side, at the anterior end of the future stomach, at about 

 the time of hatching. Their development is progressive from this point 

 backward and upward towards the dorsal side. In an embryo just hat- 

 ched, I can trace these cells along nearly the anterior half of the me- 

 dian ventral line, and farther back I find free nuclei in the surface of 

 the yolk. In the median dorsal line, I find no fully formed entoderm 

 cells (except salivary gland-cells), but do find free nuclei in the ante- 

 rior half. 



Sense-organs. 



The sense-organs of the lip arise as bulb -like thickenings of the 

 epidermis. At the time of hatching, long before the eyes and segmental 

 sense-organs appear, two pairs of these sense-bulbs are found, symme- 

 trically placed on the surface that is to form the margin of the lip. The 

 symmetrical arrangement in pairs, the second pair being a little behind 

 the first and farther apart, suggests that these organs were primarily 

 strictly segmental. 



Larval Gland-cells. 



The origin of these provisional gland-cells has been given above. 

 It appears to me not improbable that the epidermal thickening, from 

 which these glands arise, has misled Nusbaum into his view of the 

 origin of the nervous system. This author has fallen into the error of 

 supposing that the function of these gland-cells is to bind the embryos 

 to each other, while they attach themselves to the ventral side of the 



