W. G. YanNanie — Embryology of Eustyloehns. 299 



Fig. 18. — Sperm-niicleiis from an egg in the stage of fig. 17. x 860. 



Figs. 19a and 196. — Somewhat later stage than fig. 17. The polar spindle aster 

 has degenerated, its centrosome and centrosphere have vanished, and the 

 rays become very indistinct. The vesicles are larger and each contains a 

 nucleolus. The sperm-nucleus and several of the female vesicles appear in 

 fig. 196, which is another section of the same egg. x 860. 



Figs. 20 and 21. — Show the fusion of the vesicles into the female j^ronucleus, the 

 remains of the second polar spindle aster and the male pronucleus. The 

 latter lies unusually far from the center of the egg in fig. 20. x 860. 



Note. — Figs. 19 and 20 show the difference in the condition of the chro- 

 matin in the first and second polar bodies. In fig. 19« the first polar body lies 

 on the right, in fig. 20, on the left. 



Fig. 22. — Very early stage of the first cleavage spindle, showing the first appear- 

 ance of the cleavage centrosomes. The female pronucleus, which is three- 

 lobed, lies above; the male, below and to the right. Threads of chromatin, 

 which will develojj into the chromosomes of the cleavage spindle, are begin- 

 ning to be noticeable, x 860. 



Plate XXXIX. 



Fig. 23, — A rather early stage of the first cleavage spindle. The threads of 

 chromatin can be seen in the pronuclei, but their membranes are still com- 

 plete or nearly so. The centrosomes are not stained, x 860. 



Fig. 24. — First cleavage spindle, shortly after the disappearance of the pronu- 

 clear membranes, which have dissolved while the pronuclei were still some 

 distance apart. Two groups of chromosomes are shown, derived from the 

 male and female pronucleus respectively. The centrosomes are not stained 

 and the centrospheres are poorly preserved, showing an outer ring not 

 normally present, x 860. 



Fig. 25. — First cleavage spindle fully formed, x 860. 



Fig. 26. — Late stage of the same showing the centrosomes of one pole only. 

 The single centrosome shown in each pole in fig. 25 has divided into two, 

 which separate in a direction transverse to the axis of the spindle. The 

 centrosphere is more differentiated than in the earlier cases, as it is in the 

 later stages of the polar spindles also. x 860. 



Figs. 27a and 276 — Early stage of the second cleavage spindles. One aster, 

 lying in the next section, is shown in fig. 276. x 860. 



Plate XL. 



Fig. 28. — Second cleavage spindles. The XJ-form of the chromosomes can be 



seen in the right hand blastomere. The centrospheres exhibit the same 



defect described in fig. 24. x 860. 

 Fig. 29. — Two cells of an egg in the four cell stage with third cleavage spindles. 



x860. 

 Fig. 30. — Process of forming the first polar body. Outlines of the forma 



assumed by the polar body in one instance in the course of ten minutes. 

 Fig. 31. — Outline of a spermatoplnu'e. The small part only is inserted througli 



the opening made in the epidermis. 



