EXPLANATION OF THE PLATES. 39 



PLATE XXIV. 

 Magnification = 280. 



Fig. 1. Surface view of the blastodisc about five minutes after the first appearance of the cleavage am- 

 phiaster, forty-five minutes alter fertilization. 



Fig. 2. A median vertical section of a ilisc of the same age, in which the equatorial nuclear plate is in 



process of division. 

 Fig. 3. A similar section at fifty minutes, showing the nuclear plate divided. 

 Fig. 1. Another disc of same age, but ce advanced, in which are seen two distinct nuclear spheres, each 



with an equatorial plate. 

 Fig. 5. Surface view of yolk elements seen beneath the disc in Figs. 2-4. 

 Fig. (i. Surface view of blastodisc fifty minutes old, iu which the amphiaster appears in the form of a 



conspicuous dumb-bell-shaped figure. 

 Fig. 7. Vertical section of the same. 



PLATE XXV. 



Magnification = 280. 



Fig. 1. Blastodisc in an early stage of cleavage. Although no groove is visible, the line of division is pre- 

 marked by minute vacuolar areas. The chromatic elements are still far removed from the astral 

 centres. 



Fig. 2. The first cleavage is not yet completed ; but the periplasts have already elongated at right angles 

 to the first plane of division, and the nuclei have reached the centres, where they are already 

 preparing to divide. The second plane of cleavage is only obscurely marked in the left blas- 

 tomere, and barely recognizable iu the middle of the right. 



PLATE XXVI. 



Figs. 1-4. Optical sections of the blastodisc of an egg fertilized artificially, after it had lain in water about 

 five minutes. X 120. 



Fig. 1.. Twenty minutes after fertilization, the first polar globule had not appeared, but it was probably con- 

 cealed in the micropyle. The disc is suspended by a point to the micropyle, but is elsewhere 

 separated from the membrane by a broad perivitcllinc space. 



Fig. 2. Thirty minutes after fertilization, the first polar globule appeared at the mouth of the micropyle. 



Fig :i. Forty-live minutes after fertilization, the disc snapped its connection with the micropyle. and the 

 polar globule was suddenly thrown at a little distance from the egg. The disc, which had 

 already begun n> flatten on its inner surface, and to rise towards the membrane, at once as- 

 sumed a perfectly symmetrical form, and five minutes later (Fig. 4) the inferior groove of the 

 first cleavage appeared. 



Fig. 5. A section of one of the blastomeres of the two-cell stage, in the plane of the dividing nucleus. The 

 two new nuclear spheres have already separated, leaving a clear zone iu the middle, in which 

 the nuclear plate remains still undivided. The centrosomes of the asters were faintly 

 stained. X 280. 



Fig. da-f. Successive horizontal sections of (lie two-cell stage, beginning at the level of the nuclei, which 

 are already in process of second division, and ending with the inner face of the disc. The 

 elongated oval outline at the centre of (/ is the upper part of the inferior groove with which 

 the second cleavage begins, and b-f show the form and extent of the groove at successive 

 levels. X 2S0. 



Fig. 7. The second cleavage, begun in Fig 5, is here completed, and is seen to pass quite through the 

 disc. X 280. 



Fig. 8. The boundary lines of the blastomeres in the S-ecll stage, with peculiar vacuoles on either side 

 Notice the congruence of diagonally opposite blastomeres, a a', b b', cc, and <l d '. 



