MEMOIRS OF THE NATIONAL AOADKMV OF SCIENCES. 457 



is differeiitiiited lioiii a distinct epitlielial cell, the nucleus of the cell lieconiiii;>: tlu" nucleus or 

 geiininal vesicle of tlie egn. Some of the epitlielial cells euwiap the (leveIo[)in;; 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 fullicular (U'lls. 



(7) In Ilomarus and I'alinurus the character of the germinal epithelium is somewhat <lifferent 

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

 extends inward from the wall, i)i the form of radial sheets or folds, between which are rei-ntrant 

 blood vessels. There aie a number of germogenal areas corres])on(ling to the folds in which the 

 ova originate. During growth the eggs gradually pass from the center toward the perii»hery. 

 In the germogen the cell outlines are obscured.* 



(8) The yolk arises within the cell protop|asm, and in Homarus degenerating nuclei occur in 

 the ovarian stroma, 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 chara<-terize the mature ovary. They appear to have a dire<'t 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) I have observed a single ]<olar body in a sec.lion of the egg of Stenoi)US, in which the male 

 and female i)ronuclei were present, and two ]>olar bfidies in the ripe nncxtruded egg of the lob- 

 ster. In lobster's eggs also which failed of extrusion at the proi)er time, an<l which eventually 

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

 female pronucleus. It is thus probable that the polar bodies are often, if not always, given oft' 

 before the eggs are laid, t 



Segmentation in Alpheun minor. — (10) The segmentation in Alpheux minor is in some respects 

 anomalous, and the conclusion seems to be warranted that we have here a case of amitosis, unlike 

 anything wlii(;li has been hitherto described in Crustacea. Unfortunately my material is not at 

 present suflicient to enable lue to say in exactly what way the usual i)roce8s of cell division has 

 here been uio<litied. 



Delamination . — (II) The segmentation has been thoroughly reviewed in Section v, and 

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

 the close of segmentation in the lobster .some of the blastodermic cells delaminate ami their 

 products pass into the yolk. In Alpheim snulcyi a similar migration of cells from the sui>crticial to' 

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

 was preceded by delamination or not. These cells api>ear to originate in greatest number over 

 that side of the egg which corresponds in position to the embryonic area. It seems i)ossiblc that 

 these cells may represent a primitive endoderm, the function of which has been usurjied. In the 

 lobster they speedily degenerate. 



Invagination tStage. — (12) The invagination 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 n\emoir the ventral 

 or thoracic-abdominal i)late. Cells also continue to i)ass into the yolk from the ventral plate. 

 While cells are constantly being subtractcil from the ])late, it is <!onstantly receiving new recruits 

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

 a multitude of migrating (tells, derived originally from three .sources: from the blastoderm, from 

 the cells which are first invaginated, and from those which originate later from the ventral ]>late, 

 after all trace of the superficial pit has (lisai)pcared. 



Oerm-laycrs. — (13) The.se migrating cells, which are collectively called "the wamlering cells" 

 in Section vii, spread to all parts of the egg. While it is i)erfectly obvious that these bodies 

 represent mesodennic and cnd()(b'rmi<! tissues, it is not so easy to determine what i)articui;ir cells 

 give rise to this or that layer, nor iS it easy to decide in many ca.ses, from a superficial study, wliether 

 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 Las been recently described by Uunipus in a detailed pai)er upon the 

 embryology of this species. Ho has culled .itteution to the folded character of thi! ovarian epitludiiiin, which is so 

 marked in the young or immature ovary. (The Embryology of the Americau Lobster, by Hcniioii Carey linnipus, 

 Jourii. of ilorplioloijij, Vol. v, No. 2, 1891.) 



t Polar bodies have beeu recently described in the external eggs of the lobster by Bunipus. Op. cit. 



