MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



423 



cau be said. Brooks thought that in Lucifer they represented the food yolk, although this was 

 not settled. In Alpheus these cells are later joined by grer.t numbers of wandering cells at the 

 invagiuation period, aud out of this common stock, so far as we can determine, both mesoblastic 

 and entoblastic organs are developed. In Hoiuarns, on the other band, the iuvagiuate cells 

 unquestionably degenerate. 



The eggs of Scyllarus and PaKmirus have not been thoroughly studied, so far as I am aware. 

 Dorhn (14), however, has figured the eggs of Scyllarus arctus in late stages of yolk segmentation, 

 and in surface view they resemble, at this time, the ordinary Decapod type. We have as yet no 

 knowledge of the segmentation of the Stomatopods. 



The relations of the different varieties of segmentation which are met with among the Deca- 

 pods, may be expressed by the following table : 



Segmentation of the 

 Decapod egg. 



I. Total : regular : seg- 

 mentation cavity pres- 

 ent. 



II. Partial : centroleci- 

 thal (yolk pyramids): 

 segmentation cavity 

 filled with yolk. 



(I). Lucifer typus. 



Segmentation of yolk at first to- 

 tal, afterwards partial. 



(1) Palermo*!. 



(2) Eupagurus prideauii., 



(3) Atyephyra compressa. 



(1) Penunis. 



(2) Crangon. 



(3) Stenopus hiapidug. 



rRegula 



(II). Segmentation of I 

 yolk. 



I Irregular 



(5) Pontonia. domestica. 



(6) Hippo, talpoides. 



(7) Palirmonetes vulgaris. 



1. (8) Cnllianassa mcdi/erranea. 



(1) Homarus americanus (yolk 



Begmentatiou at first ir- 

 regular, but later regular 

 or nearly so). 



(2) Alpheus minor. 



Of the Thoracostraca, the Schizopods undoubtedly depart widest from the common decapodal 

 .type of segmentation. Nusbaum (45) thus describes the process in Mysis Chameleo : The pro- 

 toplasm that is, the. segmentation nucleus, with its protoplasmic body retreats to a point at the 

 surface of the egg. The nucleus then suffers division, aud the protoplasm becomes differentiated 

 into an outer striated zone containing a single nucleus, and an inner granular zone with one or more 

 nuclei. The single external necleus divides and gives rise to a small blastodermic disk, formed of 

 a single layer of hexagonal cells. From the internal nuclei aud protoplasm a small number of nuclei 

 are produced under the blastodermic disk. The free cells below the disk are the products (1) of 

 the nuclei and formative protoplasm of the deeper layer, and (2) of the cells of the upper layer or 

 blastodermic disk. While a solid accumulation of cells is thus being formed below the disk, the 

 superficial cells gradually extend on all sides and inclose the egg. The thickened disk marks the 

 ventral side of the embryo. It divides into a median, caudal, or abdominal plate, and two lateral 

 plates, the ventral bands. Yolk cells which were not present up to this stage, now arise by migra- 

 tion from the abdominal plate. 



The segmentation of the Schizopod is especially interesting, since it agrees so closely with that 

 described in certain Isopods and Myriapods, and resembles also the segmentation of Arachnids. 



Bobretyky's observations on Oniseus (5) need to be repeated, and especially in the early 

 stages. According to this observer the earliest phase of segmentation was characterized by the 

 anomalous withdrawal of the egg protoplasm to the surface, where it accumulated in a distinct 

 body, and underwent segmentation. A disk of large columnar cells was thus formed, marking the 

 ventral surface of the embryo. The cells spread with the thickening of the disk, until they inclose 

 the entire egg. The superficial cells form ectoblast, the rest entoblast and mesoblast. 



Morgan (41) has described a form of segmentation in Pycnogonids which resembles that of the 

 Decapod. Here the invagination (which leads to the formation of the stomoda-um) is preceded by 

 the delami nation of endoderm cells from the blastosphere, very much as iu Alpheus, if we may 

 regard the yolk cells as primitive eudoderm. 



