THE AMERICAN LOBSTER. 203 



single globule of yolk is practically colorless, and as I have never detected the polar 

 bodies in stained sections I can not affirm that the small particles which seemed to 

 answer to their assumed appearance were not detached globules of yolk. In ovarian 

 eggs which had failed to pass out of the body at the time of ovulation I have seen 

 what looked like polar corpuscles, but here, although the nucleus of the ovum was 

 at the surface, the observation could not be confirmed by histological analysis. The 

 position of the nucleus in such cases seems to point to the extrusion of the polar cells 

 under normal conditions, while the eggs are within the ovary or its ducts. 



I have already described and figured the egg of the shrimp, Stenopus hispidus, 

 iu which two cells and a single polar body can be distinguished in sections (95, fig. 

 1, plate G). One cell lies at the surface, and very near it in the space beneath the yolk 

 and shell a spherical mass of deeply staining chromatin, corresponding in size with 

 the nucleus of the superficial cell. It is probable that the latter represents the ger- 

 minal vesicle after one division, and that the deeper lying cell is the male pronucleus. 

 (Compare 94, description of plate, p. 474.) 



THE EXTERNAL PHENOMENA OF SEGMENTATION. 



A colored sketch of the fresh eggs of the lobster is given in fig. 24, plate 17. These 

 were laid in an aquarium, and when examined August 11, 1893, were closely adherent 

 and could be separated only with difficulty. The fresh egg is spherical, oblong, or 

 somewhat irregular iu form, and measures about -£■ inch in diameter. (See p. 55.) It 

 has in appearance a fine granular texture all over, owing to the uniform distribution 

 and character of the yolk spherules, and the shell hugs the egg closely in all its parts. 



An early sign of development is the flattening of a part of the surface of the 

 yolk and the consequent elevation of the shell over this area. A liquid, in which a 

 granular substance is sometimes feebly developed while the egg is still fresh, is 

 pressed out of the yolk and fills the free space between it and the shell. This flat- 

 tened area marks the animal pole of the egg and is very characteristic. The surface 

 of the ovum is often flecked with light spots due to the irregular grouping and per- 

 haps looser arrangement of the yolk spherules. Light flecks, three to four iu number, 

 but of different character, now appear in the depressed area (fig. 215). These are cells 

 which are approaching the surface, and their nuclei can now be seen shimmering 

 through the green yolk. 



The phases which immediately follow are represented in figs. 216, 217, and 218, 

 which were drawn from the same egg at successive stages of development. The cells 

 approach nearer to the surface, multiply by indirect division, diffuse about the animal 

 pole, and bring on the superficial segmentation of the yolk into hillocks as seen in profile 

 in fig. 218. The drawing shown in fig. 216 was made at 10.30 a. m. At the animal pole 

 there are seen two double rows of cells, 8 in each double row, or 16 in all. These are 

 arranged in pairs — four pairs of daughter cells in each double row — the products of 

 recent division. This egg appears in profile iu fig. 218. The yolk is now in contact 

 with the shell over less than half its area, but the yolk hillocks appear about the 

 animal pole only. At 10.55 a. m., 25 minutes later, 20 cells could be detected. At 12 

 m., or 65 minutes later, this egg had the appearance shown in fig. 217. The segments 

 or yolk hillocks were then farther apart. This process continued until the entire 

 surface of the yolk was segmented. 



Opposite sides of the same egg in which that condition was already realized are 

 represented in figs. 219 and 220. The former shows the animal pole, the latter the 



