MEMOIRS OF THE NATIONAL ACADEMY OK SCI ENCKS. 



397 



In insects it appears that a clioriou is always present in ovarian eggs, while, on the other 

 hand, arachnids possess a vitelline membrane and the eggshell is secreted in tlie oviduct. 



.There is no simple rule to express the appearance of egg membranes in a diversified group 

 like the Arthropods, and, considering that these structures are purely secondary to the cell and 

 expensive products however formed, this is what we should expect. Their function is chiefly pro- 

 tective, and where a choriou is present in the ovary a yolk membrane is not developed, but the 

 latter is present, as in spiders, when the shell is a later product. Ercll (15) describes three egg- 

 membranes for the lobster, but it is clear, as Mayer lias already shown, that the inner, delicate 

 membrane which has been described for the decapod egg, is a secretion product of the blastoderm. 



III. SEGMENTATION OF THE EGG OF ALPHEUS MINUS. 



In the small green Alpheus of the southern coast we have a peculiar modification of the usual 

 process of segmentation, which seems to be anomalous. 



The fertile egg- is pervaded wi'th a remarkably fine reticulum, which incloses \<>lk spherules 

 of minute and uniform size. The nucleus is central, or nearly so,* and consists of an ill-defined 

 mass of protoplasm, in which a fluechromatiu network is suspended. In the next phase (I'l. xxvr, 

 Fig. 14) the nucleus is elongated and about to divide. Division appears to bo direct and irregular. 

 At a somewhat later stage the phenomena of the most interest occur (Figs. 12, 13). Each product 

 of the first nucleus has developed a swarm of nuclear bodies (S. S.), which seem to arise by fragmen- 

 tation. These bodies take the form of spherical nuclei in clear masses of protoplasm. The yolk 

 frequently has a tendency to segment about the nuclear masses, in the same way that it divides 

 about a single nucleus to form a yolk pyramid. This yolk segmentation seem.s to be normal, but 

 it is very irregular. lu one case there were two large segments, whicb nearly divided the. egg in 

 two, besides several smaller ones. Nuclear matter consists either of small particles or of indefi- 

 nite reticulated masses, resembling the first nucleus (Fig. 14). Clear areas are sometimes found 

 with nuclei which appear to be breaking down. About eight nuclear swarms or clusters are present 

 in the stage shown in Figs. 12, 13. The nuclei vary in size from refriugent particles to bodies of 

 ordinary nuclear appearance. 



Figs. 25 and 20 represent two sections of one of the clear areas in the same egg from which 

 Fig. 12 was drawn. This clear field has several degenerating nuclei near its border. The largest 

 one (S. C.) is included in both sections. A small chromatiu mass with indistinct body lies next 

 it (S. C. 2 ), and other similar bodies occur in different sections. The cell S. C. contains two chro- 

 matiu balls, and in Fig. 26 (the next section but one in the series) this body appears to be dis- 

 charging through its broken-down wall numerous minute elements (S.) into the clear field. In 

 Fig. 22 a small protoplasmic area occurs, in which a single nucleus lies. This body is granular 

 and contains a large chromatiu ball. Figs. 5 and 23 are also from the same egg. Here we see 

 structures similar to the cell just mentioned. They are surrounded by yolk and consist of a deli- 

 cate reticulum in which usually one large uucleolus is suspended, besides great numbers of small 

 chromatin particles. 



Various stages of growth are here represented, and it might appear at first sight that we have 

 a case of endogenous cell formation. I see no reason to suppose that the eggs examined are abnor- 

 mal, and I conclude that we rather have in this species a remarkable modification of the usual 

 indirect cell division, attended by an equally remarkable degeneration of nuclear material. 



In the last stage obtained (Fig. 29) the whole egg is filled with several hundred very large 

 elements, which are descended more or less directly from some of the nuclear bodies just consid- 

 ered, but the intermediate stages have not been traced. This probably corresponds to stage vi ol 

 A. sanlcyi, at the period just before invagiuatiou, but it is quite unlike anything which I have seen 

 in other species. The yolk is now irregularly segmented into blocks or balls, but probably not 

 with reference to these cells. 



This case is interesting when we compare it with the degeneration of cells to be described in 

 another section, and from a cytological point of view it deserves careful study. 



* In single sections the nucleus is strictly central, but whether it is NO with respect to the entire, e^y it is imi easy 

 to determine. Minot states that the egg nucleus is always eccentric. Am. Saluni/ixt, Vol. xxm, 1889. 



