398 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



IV. — THE DEVELOPMENT OF THE EMBRYO. 



Stage I. — Segmentation to formation of the blastoderm. 



These observations relate to Alphevs heterocheUs of the Southeru States, to a Bahaman form 

 which hatches as a zoea but which otherwise resembles this species very closely, and to Alpheus 

 mulcyi, also the from Bahamas, which has large eggs and a nearly direct development. Except 

 where it is necessary to mention specific differences, these three species will be treated as one form. 



In June two Alphei {A. saulcyi) laid eggs in an aquarium, but the ova were in each case 

 unfertilized, and for the most part failed to adhere to the swimmerets. One of these eggs, hardened 

 at an interval of five hours after it was laid, is shown in section on PI. xxvii, Fig. 17. I regard 

 the nucleus of this egg as the female pronucleus. It consists of clear protoplasm, which stains 

 ieebly and sends out processes on all sides into the yolk, and of an indefinite chromatin networic 

 susi)ended in it. The massive yolk is composed of corpuscles of uniform size, excepting at the 

 periphery where they are much smaller. Numerous small lacunae occur, representing jiarts of the 

 yolk which were soluble in the reagents employed. The chorion, or inner egg-membrane, is 

 transparent, tough, and very distensible. It frequently splits into thin layers when subjected to 

 the hardening and embedding process, thus showing the manner in which it is formed in the 

 egg-follicle. It thus appears that the unfertilized egg of Alpheus is incapable of segmentation. 



The first segmentation nucleus has been observed in a few cases. That shown in Fig. 16 is 

 possibly preparing for division. It possesses a fine reticulum ; it is lenticular in shape, and granular 

 in appearance, and is surrounded by protoplasm which spreads into the yolk. The ouce divided 

 nucleus and the next phase with four cells were not obtained in Alpheus, but the latter was seen 

 in an allied prawn (Pontonia domestica}, and is shown in Fig. 27. One of the three cells present is 

 in the aster stage of karyokinesis and has a well-marked equatorial plate. The third segmenta- 

 tion phase is illustrated in Figs. 9, 28, and 30. In the section through the entire egg, three of the 

 eight cells present are met with, and one of these (x) is shown with greater detail in Fig. 30. A 

 cell in process of division is represented in Fig. 28. In another egg with eight cells present, two 

 are undergoing division in different planes. As before, the cells consist of a chromatin network 

 of various shapes surrounded by a clear protoplasmic body, which sends out processes between the 

 surrounding yolk spheres. It is important to notice that the products of the segmentation of the 

 first nuclr^us pass gradually and uniformly to the surface of the egg. At this stage they have not 

 reached the surface but are visible through the egg shell. The yolk in these specimens consists of 

 spherules or angular blocks (Fig. 28, Y. S.), which are largest in the center of the egg, and con- 

 tain very few vacuoles. 



The fourth phase of segmentation is attended by the cleavage of the superficial i)arts of the 

 yolk (Fig. 10) around the nuclei, thus giving rise to sixteen blastomeres or partial yolk i)yramids. 

 The division of the yolk proceeds but a short distance below the level of the nuclei, so that all the 

 yolk-pyramids open by their inner ends into the common yolk mass which fills the segmentation 

 cavity of the egg. The base of the pyramid, which abuts on the surface is polygonal in shape, and 

 at its middle point some distance below the surface, the nucleus is seen with its investments of 

 protoplasm. The nucleus is large and granular, and the protoplasm which surrounds it reaches 

 out on all sides into the yolk. We may look upon the yolk pyramid as a cell in the strict morpho- 

 logical sense, its protoplasm being concentrated about the nucleus. The blastoderm or primitive 

 egg envelope arises through the multiplication and consequent reduction in size of these huge 

 yolk elements. The surface has then the usual appearance of a fine mosaic of hexagonal i)lates 

 or cells, the nuclei and surrounding protoplasm of which lie at the surface. 



The fifth segmentation phase is shown in Figs. 15 and 31. The septum between the pyramids 

 extends farther into the yolk, while the nuclei are slightly nearer the surface, and the long axis of 

 each is parallel with it. This particular egg was taken in a period of " rest," but in others the 

 nuclei are in karyokinesis, the division being always radial or in a plane at right angles to a 

 surface tangent. 



The segmenting egg of Hippa talpoides is shown in Fig. 1 (32-cell stage;, and a section of a 

 later phase in Fig. 4. The egg appears to have undergone a total segmentation when seen 

 from the surface, but this is not quite so marked as represented in the sketch. The yolk pyramids 



