458 MEMoms OF the national academy of sciences. 



ject is fully considered in Section tii, the general results being : (1) That it is not possible to 

 decide what part the primary yolk cells play in Al^jhens, for reasons which have been already 

 considered; (2) that the great bulk of the cells which migrate forward from the area of invagina- 

 tion and attach themselves to the embryo, or proceed to the peripheral parts of the egg and take 

 up a position at the surface, are undoubted mesoblastic elements ; (3) tliat those cells which give 

 rise to the eudodermal epithelium in the egg uauplius are derived largely from cells which migrate 

 in a posterior direction from the area of invagination ; (4) that degeneration, followed by the death 

 and dissolution of the chromatin and cell protoplasm, is characteristic of the wandering cells at 

 about the beginning of the egg-nauplius iJeriod. The mesoblast has become a well-recoguized 

 layer before the endodermal epithelium has appeared. 



(14) The egg, with centrally moving cells whi(;h have budded from the blastoderm, may be 

 compared with the j>Z««Mia stage of Cffileuterates, and the internal cells may represent the primi- 

 tive eudoderm. According to this view, the invagination stage has no reference to an adult 

 gastrula-like ancestor, but is a purely secondary condition, which became so impressed upon the 

 ancestors of the present Decapods that it has remained in their ontogeny. 



In the majority of Decapods which have been studied the invagination has no direct relation 

 to the mouth or anus, or to the alimentary tract. The conditions which are present in the cray- 

 fish cannot be regarded as typical or primitive. 



(15) in Alpbeus and Homarus the primitive mouth arises on a line between the rudiments of 

 the first pair of antenna, but these appendages are never jjost-oral. The hind gut originates as a 

 nearly solid ingrowth, apparently at a point considerably behind the position of the pit due to the 

 first invagination, and is formed one or two days later than the mouth. 



Cell, dissolution. — (16) The degeneration of embryonic cells is treated at length in Section vi. 

 It is remarkable that the early segmentation stages of Aljyheus minor are attended with the degen- 

 eration of protophism. The chromatin residues remain for some time in the yolk, and eagerly 

 react upon dyes, but gradually lose this power and eventually enter into the general nutrition. 



(17) Degenerating cells appear in greatest force in Alpheus, Astacus, and Homarus at about 

 the egg-nauplius stage, and from that time their numbers begin to wane. They appear in one 

 instance before the differeutiation of the germinal layers, and are not confined to any one layer at 

 a later period, but in Alpheus scntlcyi they are most characteristic of the wandering cells, which 

 repi'esent mesoderm and endoderm. The "secoudary mesoderm cells " and " white-yolk elements" 

 which have been described by Keicheubach, are to be regarded as degenerating cells. Degenera- 

 ting cells also occur in connection with the " dorsal phite." 



The Eyes, — (18) The details of the structure and development of the eyes and nervous system 

 are fully reviewed in Sections Tin and ix. 



The eyes and optic glanglia are derived from the optic disk, in the formation of which there is 

 in Alpheus no proper invagination. The thickening of the disk is accomplished by emigration 

 from the surface and by the delamination of superficial cells. An area of active cell division can be 

 distinguished, which corresponds to the invagiuate area of the optic disk of the crayfish. The 

 cells which migrate from the center give rise to the rudiment of the optic ganglion. The disk 

 grows out in the form of a lobe, and becomes differentiated into an outer retinal layer and an inner 

 ganglionic layer. The eye proper is differentiated from the retinogeu, which is primitively a single 

 layer of ectodermic cells. 



(19) I am inclined to regard the "compound eye" not as an aggregate of simple eyes, as its 

 name implies, each of which is due to a hypodermal infolding, but rather as a collection of differ- 

 entiated clusters of ectoderm cells, originating in a single epithelial layer. 



(20) The absence of light has no appreciable effect on the development of the eye pigment, 

 but in Palwmonetes the distal retiunlar cells respond very promptly to the action of light. If the 

 light is excluded from the eye, these cells migrate outward and enshroud the proximal ends of the 

 cones, sending out pseudopodal prolongations to the cornea. When the eye is again stimulated by 

 light the pigment immediately retreats from the surface, and the cell takes the form of a plaited 

 black ribbon, leaving the cones free. 



Adelbert College, , 



