MEMOIRS OF THE NATIONAL ACADKMY <>K SCIENCES. 



457 



is differentiated from a distinct epithelial cell, the nucleus of the cell becoming the .nucleus or 

 germinal vesicle of the egg. Some of the epithelial cells enwrap I he developing ovum and form 

 the follicle, or pocket in which it is lodged. The chorion or inner egg membrane is the. direct 

 secretion product of the follicnlar cells. 



(7) -In Homarus and Palinurus the character <>f the germinal epithelium is somewhat different 

 from that of Alpheus. The outline of individual cells is obscured and the germinal epithelium 

 extends inward from the wall, in the form of radial sheets or folds, between which are reentrant 

 blood vessels. There are a number of gerniogenal areas corresponding to the folds in which the 

 ova originate. During growth the eggs gradually pass from the center toward the, periphery. 

 In the germogeu the cell outlines are obscured.* 



(8) The yolk arises within the cell protoplasm, and in Ilomarns degenerating nuclei occur in 

 the ovarian stronui, and it is probable that a certain number of nuclei degenerate and enter into 

 the food yolk of the egg. In the lobster also some of the follicle cells develop into gland-like struc- 

 tures which eharacterix.e the mature ovary. They appear to have a direct relation to the growing- 

 eggs, but their true significance has not yet been ascertained. In about two weeks after the eggs 

 have been extruded these structures have almost wholly disappeared. 



(9) 1 have observed a single polar body in a section of the egg of Stenopus, in which the male 

 and female pronuclei were present, and two polar bodies in the ripe unextrnded egg of the lob- 

 ster. In lobster's eggs also which failed of extrusion at the proper time, and which eventually 

 degenerate in the ovary, I find that the nucleus is at the surface. It has the appearance of a 

 female pronncletis. It is thus probable that the polar bodies are often, if not always, given oil 

 before the eggs are laid, t 



Segmentation in Al/iln'/i* ininor. (10) The segmentation in AlpltrH* minor is in some respects 

 anomalous, and the conclusion seems to be warranted that we have here a cast.- of a mitosis, unlike 

 anything which has been hitherto described in Crustacea. Unfortunately my material is not at 

 present sutlicient to enable me to say in exactly what way the usual process of cell division has 

 here been modified. 



Delamin&tion. (11) The segmentation has been thoroughly reviewed in Section v, and 

 it is unnecessary to repeat the details. 1 wish to call attention, however, to the fact that at 

 'the close of segmentation in the lobster some, of the blastoderinic cells delaininate and their 

 products pass into the yolk. In Alfil/i'iifi mtiilci/i a similar migration of cells from the superficial to 

 the deeper parts of the egg occurs, but in this case it was not determined whether this migration 

 was preceded by delamination or not. These cells appear to originate in greatest number over 

 that side of the egg which corresponds in position to the embryonic area. It seems possible that 

 these cells may represent a primitive endoderin, the function of which has been usurped. In the 

 lobster they speedily degenerate. 



Imagination Stage. (12) The invaginatiou stage, which soon follows, results in the admis- 

 sion of more cells into the yolk and in the formation of an organ called in this memoir the ventral 

 or thoracic-abdominal plate. Cells also continue to pass into the yolk from the ventral plate 

 While cells are constantly being subtracted from the plate, it is constantly receiving new recruits 

 from the surface, owing to the activity of cell division in this region. \Ve thus have in Alpheus 

 a multitude of migrating cells, derived originally from three sources: from the blastoderm, from 

 the cells which are first invaginated, and from those which originate later from the ventral plate, 

 after all trace of the superficial pit has disappeared. 



Germ-layers. (13) These migrating cells, which are collectively called > the wandering cells" 

 in Section VII, spread to all parts of the egg. While it is perfectly obvious that these bodies 

 represent rnesodermic and endodermic tissues, it is not so easy to determine what particular cells 

 give rise to this or that layer, nor is it easy to decide in many cases, from a superficial study, whether 

 migrating cells may not be derived from the ectoderm in various parts of the embryo. This sub- 



'The structure of the ovary of the lobster lias been recently described by JJumpn.M in a .kt.-iilr.il paper upon thu 

 embryology of this species. He lias called attention i.. tin- folded character of the ovarian epithelium, which is so 

 marked in the youn'g or immature, ovary. (The Embryology of the American Lobster, by Ili-nuon Carry Humpns, 

 Jovrn. of Morpholoyy, Vol. v, No. 2, 1891.) 



t Polar bodies have been recently described in the . xierual eggs of the lobster by Bumpus. <>],. cit. 



