11)07.] NATURAL SCIENCES OF PHILADELPHIA. 559 



Fig. 29. — Similar vievr of sliKhtly older egg, showing preceding ^divisions 

 completed and the resulting small superficial cells la»-i-2-2-'_id'-'--^'. 



Fig. 30. — Egg from near the vegetative pole, showing division of 4d' com- 

 pleted and budding in of the cells 26'-' and 2d'-'. 



Fig. 31. — Egg from animal pole at slightly later stage than fig. 29. Shows 

 divisions of 4cP' and 4:d--\ The resulting sniall cells 4cP'-' and 4cP''-^ 

 are probably entoblasts. 



Fig. 32. — Optical section from animal pole, showing the cells 2a|'-2d'-* 

 budding mesoderm to the interior. Also shows the large nuclei of 46, 

 4a and 4c. 



Fig. 33. — Later stage, showing the internal divisions of the entoblasts 4d' 

 and 4d\ Not all the cells on the interior of the egg are shown. 

 Plate XL. — Fig. 34. — Optical section of egg from the posterior side. The 

 entoblasts 4d'-* and 4d'-^ have di\dded once 'and possibly twice. The 

 two small entoblasts 4fF-'-i and 4fp-2-' are shown vidth dark nuclei. 

 Aliove these are the two small mesoblasts, while the larger mesoblasts 

 4f/2-'-^-2 and icP-''-'^-^ are again dividing. Above these are a number of 

 cells from the first quartet which later form the ganglion. The large 

 nuclei of 4a, 46 and Ac are shown in dotted outlines. 



Fig. 3o. — Optical section of an egg \iewed from near the vegetative pole. 

 The derivatives of 4.(P are stippled. The division of 4(P-'-^-' and 4cP-2-2-» 

 indicated in fig. 34 are now completed and the beginning of the meso- 

 dermal bands is evident. In many eggs the divisions are not so regular 

 as sliOA\Ti here. Instead the mesoderm tends to form clusters of cells 

 rather than bands. 



Fig. 36. — Optical section of a much later stage. From the left side. The 

 mass of endoderm derived from 4d' Ues just above the future pharynx. 

 The mesoderm cells have passed to the periphery and the centre of the 

 egg is filled with the homogeneous yolk spheres (?/), in wMch the shrunken 

 nucleus of 46 can be seen"(M.46.). The two small cells marked (e) are 

 probably the entoblast 4d^-^-^ and 4d'-^-^. ' 



Fig. 37.— Optical section of an older embryo. From the posterior side. 

 '■' Shows ectodermal pharynx and beginning of lumen in the endoderm. 



Fig. .3S.— Actual longitudinal section of a still older embryo, showing the 

 backward turn of the aUmentary canal in this stage. 



Fig. 39. — Longitudinal section of a MuUer's larva. Nearly all the yolk 

 spheres have disappeared by this time and the ahmentary canal is 

 greatly enlarged but still unbranched. 



