MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 399 



(Fig. t, Y. P.) agree with those in AlplicHS and are ]»robal)ly formed in a siniihir way. In Paltv- 

 inoHctcs I'ulgaris the history of segmentation appears to be essentially the same. The nucleus 

 and biuse of one of the yolk i)yramids of this form is shown in Fig. 24. Ilerewe see that the peri- 

 nn<-lear jjiotoplasin has a rayed ai)pearance, being produced in all directions into very delicate 

 threads which ramify among the yolk spherules. Some of these threads moreover unite witii a 

 thin septum (Sep.) which forms the boundary wall between two adjacent yolk pyramids. 



Segmentation proceeds with a regular rhythm up to the fifth stage, but beyond this it soon 

 becomes irregular. A blastula is thus formed, consisting of a single layer of cells or hiastoderm, 

 and the inclosed central yolk. All the nuclei reach the surface and take part in forming the 

 blastoderm, so that all the protoplasm of the egg which is at first central or internal, comes gradually 

 to assume in the course of seguientatioii an external position with respect to the food yolk. The 

 blastodermic cell is the direct descendant of the yolk pyramid. It is improbable that the yolk con- 

 tains any active i)rotoplasm, excei)ting that which radiates from the nuclei, and which is descended 

 from the perinuclear protoplasm of the first segmentation nucleus. 



Stage II.— The blastoderm and invagination. 



The prawn when discovered with eggs in the fifth stage of segmentation (Fig. 1.5) was kept 

 in an a<iuarium, and the ova were preserved at intervals of several hours. Thus it has been 

 possible to follow the changes which take place between segmentation and invagination with 

 considerable detail. 



V The egg represented in Fig. 47 is about 15 hours older than last described (Fig. 15). Cell 

 division, which is now irregular, has become accelerated over a part of the egg so that a germinal 

 area or disk (G. D.) representing the future embryo is formed. The side of the egg shown in 

 Fig. 47 corresponds to that occupied by the germinal disk. In reverse view there are much fewer 

 nuclei. The egg has thus lost its radial syn)metry and become two sided. Invagination soon 

 follows this stage" at a certain point in the germinal area (G. D.). The superficial cells of the 

 blastoderm (Fig. 48) are about one-third their former size, but they still have the characteristics 

 of the yolk j)yramids. The cell is polygonal in surface views; the nucleus is surrounded by yolk 

 and tlie cleavage planes between adjacent cells (Fig. 38, Sep.) are still present. 



This stage is characterized by the passage of large numbers of cells from the surface to the 

 central i)arts of the egg just before the invagination takes place. This process is well illustrated 

 bj' a series of consecutive sections (Figs. 38-44) taken from the same egg. In all these sections 

 the cleavage of the yolk can still be seen. Many of the blastoderm cells (Fig. 39, a.) are in different 

 phases of division, thc^lividing plane being always perpendicular to a surface tangent. It is 

 l)robable, therefore, that the nuclei with their perinuclear protoplasm, leave the yolk pyramid 

 and pass by arufcboid movement into the interior. It is, therefore, evident that while morpholog- 

 ically the yolk pyramid is a cell, the elements which pass into the egg have also the value of 

 cells in a physiological sense. Six nuclei are met with in Fig. 38, one of which has wandered 

 some distance from the surface. In the next (Fig. 30) two cells (a. a'.) are in the aster phase 

 of division ; one (a^) has passed just below the surface, and another (a^) is near the center of the 

 egg. These cells (a, a'*, a^) are sectioned again in the following figure (Fig. 40). Various i)hases 

 of the process of migration* are seen in other members of this series. In an enlarged portion 

 of a similar section (Fig. 37) several cells are met with, some at the surface just beneath the 

 shell, and others at some distance below it. The protoplasm about the nucleus has no defi- 

 nite bounds, and is often filled with fine particles of food yolk. It is thus evident that these cells 

 feed essentially like amceba', by taking the food directly into the protoplasm of the cell. 



The critical stage at which cells begin to pass from the superficial to the central parts of the 

 yolk was obtained in an egg just thirteen hours older than the fifth segmentation phase, shown in 

 Fig. 15. There arc about a dozen yolk cells in this egg, and one of these is in karyokinesis. The 

 remainder lie very near the surface, but for the most part are 8ei)arated from it by a thin layer of 

 yolk s])liere8. It is thus clear that the migration of cells to the central parts of the egg begins 



* lu tliti lolistrr tho primary yolk cells arise l)y (Iclamiuatinii, and as suggestoil iu Section V, tbis is possibly true 

 of Alpbeus. 



