40 EXPLANATION OF THE PLATES. 



PLATE XXVII. 

 Magnification = 280. 



Fij. 1. The two-cell stage with second cleavage well advanced, seen from above. Tlie new groove is a 

 broad clear line, widening out, funnel-shaped at tlie marginal extremities. Vacuoles are seen 

 on eacli side of the first cleavage line. 



Fig. 2. The four-cell stage in process of division, seen from above. The boundary of the cleavage cavity 

 is shown by dotted lines. 



PLATE XXVIII. 



Figs. 1, 2, X 280. Figs. 3, 4, X 1060. 



Fig. 1. Vertical longitudinal sections of the four-cell stage. The preparation gave twenty-nine sections 

 (.0075""" thick). The first section hits the edges of two blastomeres, and shows how far the 

 separation from the periblast has extended. The triangular space between the blastomeres 

 and periblast vanishes in the sixth section. The nuclei are found in the eighth section. It 

 was in the tenth section that the cleavage cavity began as a vacuolar space, which gradually 

 enlarged as the sections approached the middle of the blastodise. 



Fig. 2. The two-cell stage in which the second cleavage is well advanced. Fifty-one sections. Two nuclei 

 appear in the tenth section. The subgerminal periblast appears in the twenty-fifth, and the 

 line of demarcation is nearly complete in the twenty-seventh, which is central. The remain- 

 ing nuclei appear in section 41. 



F'ig. 3. Amphiaster from 8-cell stage. Perenyi's fluid. 



Fig. 4i Another amphiaster, more advanced. Same treatment. 



PLATE XXIX. 



Magnification = 280. 



Fig. 1. Blastodise near the 8-cell stage, from below. The angles of the rectangular cleavage-cavity are 



prolonged in the direction of tlie first two cleavage planes. 

 Fig. 2. A little more advanced stage, from above. 

 Fig. 3. Transverse section of 8-cell stage through two end blastomeres. Two hours ten minutes after 



fertilization. 

 Fig. 4. Similar section through two of the middle blastomeres of another disc, showing the cleavage-cavity, 



without a periblasts; floor. 



PLATE XXX. 



Magnification = 280. 



Fig. 1. The 8-cell stage in process of division, from below. The order of the cleavage planes indicated by 



numerals, 1-3. be = blastocoele. 

 Fig. 2. Near the 16-cell stage. From above. 



PLATE XXXI. 



Magnification = 280. 



Fig. 1. Transverse sections of 8-cell stage shown in Fig. 1, PI. XXX. 53 sections, .0075 mm thick. Section 

 6 contains nuclei of two end cells ; 10-16 sections of the same cells, 16 being on the central 

 boundary. 18-21 in two middle cells, 21 falling in plane of the nuclei. These sections show 

 the periblast thinning out eentripetally. 



Fig. 2. Transverse sections of the stage shown in Fig. 2, PI. XXX. 31 sections, .00?5 mm - thick Section 

 4, in plane of the nuclei, shows only a little subgerminal periblast. In 13, in plane of the 

 nuclei of two middle cells, the periblast becomes very thin centrally-, and in 14, 15, near the 

 middle of the blastodise. it ceases t>> be continuous. 



