MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 397 



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

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



There is no sini|)le rule to express the appearance of egg membranes in a diversified gronji 

 like the Artliropods, and, considering tliat these struetnres are purely secondary to the cell and 

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

 tective, and wliere a chorion is present in tlie ovary a yolk membrane is not developed, bnt the 

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

 membranes for the lobster, but it is clear, as Mayer has already shown, jliat the inner, delicate 

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



III. SEGMENTATION OF THE EGG OF ALl'HEUS 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 with a remarkably fine reticulum, which incloses yolk s[)herules 

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

 mass of protoplasm, in whicli a fine chromatin network is suspended. In the next i)hase (J'l. xxvi, 

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

 At a somewhat later stage the j)henomeua 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 tenileucy 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 seems to be normal, but 

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

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

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

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

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

 ordinary nuclear appearance. 



Figs. 25 and 26 represent two sections of oue Of the clear areas in the same egg from which 

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

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

 it (S. C), aud other similar bodies occur in difl'ereut sections. The cell S. C. contains two chro- 

 matin balls, and in Fig. 26 (the next section but oue in the series) tbis 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 chromatin ball. Figs. 5 aud 23 are also from the same egg. Uere we see 

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

 cate reticulum in which usually oue large nucleolus is suspended, besides great numbers of small 

 chromatin particles. 



Various stages of growth aj-e here represented, aud 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- 

 mnl, aud I conclude that we rather have in this specties a remarkable modification of the usual 

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



In the last stage obtained (Fig. 29) the whole egg is tilled 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 of 

 A. saulcyi, at the period just before invagination, 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 refeience 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 i)oiut of view it deserves careful study. 



• In singlo sections tlio ouclcns is strictly central, bnt wht-ther it is so with respect to tho entire e;;K it is not easy 

 to deteruiiue. Minot states tbat the egg nucleus is always eccentric— Jni. Naturaliet, Vol. xxiii, 1889. 



