NATURAI, HISTORY OF AMERICAN LOBSTER. 323 



the lobster, but its eggs are occasionally straw color, ^grayish-green, or yellow-green. 

 When plunged in alcohol or hot water the ova respond like the shell of the animal and 

 become light red, a more stable pigment, a Upochrome, soluble in alcohol, being formed. 

 By adding alternately hot and cold water the eggs may be turned to red and green 

 several times in succession. 



The fresh-laid eggs, which are seldom seen, can be detected by examination with a 

 hand lens. The transparent capsule closely invests the yolk, which presents a very 

 fine-grained and uniform texture, quite different from that which the ova later possess. 

 Maturation is without doubt completed by the formation of polar cells either in the 

 ovary or during the passage of the eggs to the outside, although we have never been 

 able to find these bodies in stained sections of the egg. External segmentation of the 

 yolk follows in from 20 to 25 hours after oviposition, and the large yolk segments which 

 are early formed can be detected by the naked eye. A clear perivitelline space, 

 apparently filled in part with exudatian from the egg, soon appears between the shell 

 and yolk. At the close of this process, or after invagination has begun, the living egg, 

 when examined with a hand lens or low power of the microscope, is likely to be mis- 

 taken for one freshly laid. The ova, however, are not so closely adherent, are somewhat 

 lighter in color, and the yolk has a coarser and more irregular texture. The first division 

 of the protoplasm is central or subcentral. In the second and third segmentations, 

 with four and eight cells, the products begin to separate and migrate outward. The 

 greater number tend to move toward the side which marks the animal pole, where the 

 yolk becomes distinctly flattened, and the shell correspondingly elevated. The cells 

 which migrate toward the surface of the depressed area bring about the first segmenta- 

 tion of. the yolk into hillocks. As they multiply by indirect division their products 

 diffuse over the egg, and at the fifth segmentation, of 32 cells, the entire surface of 

 the yolk is thrown into hillocks or inverted pyramids. The segmentation is rythmical, 

 the early periods lasting about 4 hours, but the rythms of individual cells are not 

 in harmony, and the segments are unequal. Later when about no cells are present 

 the periodic divisions become more uniform over the entire egg. With each division 

 the protoplasm approaches nearer the surface, and meantime a limited number of cells 

 are formed by tangential divisions and migrate to the depths of the yolk. By a con- 

 tinuation of this process the yolk becomes surrounded by a thin mosiac of cells, or 

 rather by a single tier of several thousand minute columnar cells or diminutive yolk 

 pyramids of uniform size. Their "apices" blend into the central yolk mass, which 

 harbors a few wandering and degenerating cells. 



Cell division then becomes more rapid over a considerable area of the surface, 

 which includes the animal pole, and at a certain point an invagination of superficial 

 cells occurs. This begins by the in-wandering of a few cells, and is followed by the 

 rapid multiplication of those thiis immersed in the common food stock of the developing 

 egg, and by the sinking of a small area of the blastoderm about this point, forming what 

 is usually called the "egg gastrula" stage. The depression is at first shallow, and 

 becomes a well-defined circular pit, but is never very deep. It is subject ±0 marked 

 62399° — ^^ ■'■^ 



