CELL DIVISION IN EGGS OF CREPIDULA 



583 



PLATE XLIII. 



Abnormalities Found in Nature. 

 Figs. 2-5 9 11-13, 15 probably show effects of pressure; figs. 6-8 a: 



of diluted sea water. 

 Fig. 1. Immature 

 Fig. 2. First matt 



nucleus. 



abnormally large "yolk lobe" near vegetal pole, containing sperm 



Fig. 3. Enormous lobe at vegetal pole containing a sperm nucleus with sperm sphere attached; the 

 small nucleus on the right is probably an accessory sperm nucleus; the granular body near the animal pole 



is probably the egg nucleus. 



~ TCcw nrobablv distorted by pressure; egg and sperm nuclei normal but removed from animal 



Fig. 4. 

 abnormal lobe at vegetal pole 



with abnormally 



Fig. 5. 



6-8 



oppressed; two micromeres 



animal pole; karyomeres are present; probably the result of diluted sea 

 Vo-cell staee with entire amphiaster in one cell, and no nucleus or cent 



Fig. 9. Two-cell sti 



cell. Probably spindle was displaced by pressure, to one side of cleavage plane; nevertheless egg 

 with formation of well marked "Zwischenkorper." 



Fig. 10. 



chromosomes 



karyomeres in the resting stages. 



Fig. 11. Egg in which the 1st cleavage has been stopped and the nuclei, spheres and cytopl 



normal positions : probably the effect of pressure 



Figs. 12, 13. 



with 



and 



In fig. 12, the gonomeres 



plane and is constricted by it. 

 Fig. 14. Two-cell stage' 

 Fig. 15. Two cells, intei 

 Fig. 16. 



with 



with 



with 



Fig. 17. Four centrosomes, karyomeres, cleavage planes suppressed; the result, probably, of a 



tetraster. 



18-26 



Fig. 18. 



chenkorp 



PLATE XLIV. 



Abnormalities Found in Nature. 



robably of pressure; figs. 27-29, of dilute sea water. 

 „ „, „j nucleus is entirely lacking in two of the cells, though sphere and "Zwis- 

 The lobe attached to these cells indicates that the egg was subjected to pressure 



at the time of the 1st cleavage; since cells do not divide when nuclei are not present it is probable that the 

 nuclei were lost after the 2d cleavage, though there is no indication as to the manner of their disappearance. 



I 



Fig. 19. 

 Fig. 20. 



showing 

 showing a lobe in the spindle axis of one cell, the result of pressure. 



In the latter the 



Fig. 21. Third cleavage spindles are present, three in one cell, one in the other. 



die axis (Sv4) is normal, in the former abnormal: spindle one (Spl) lies at a higher level than 



two and three (Sp2, SpS). 



Fig. 22. Five macror 

 tetraster in the lower cells. 



Fig. 23. Seven macn 



abnormal 



have 



micromeres of the first set. One of the macromeres (ID 2 ) lies far from the animal pole and does not form a 



The nuclei are double or irregular in shape in macromeres 15 and ID' and also m the micro- 

 meres derived from them. 



micromere 



Fig. 24. 



animal pole where they have formed 



micromeres of the first set and are forming in lacotropic direction six micromeres of the second set; one 

 micromere ID 2 lies at a deep level and forms no micromere; a triaster is present in IB, a double nucleus in 

 ID 2 and double nuclei and spheres are found in the micromeres 16 and Id. . . 



Fig. 25. Six macromeres (2A-2D), five of which are dividing in dexotropic direction to give rise to 

 five micromeres of the third set. There are six micromeres of the second set and twelve of the nrst with ft 



ontaining two nuclei unaccounted 



unequa 



as m normal v, &6 o. . , 



Fig. 26. Three macromeres, one of them (D) giving rise to a micromere of the first set in normal 

 direction; the other (3£, 3C) have produced micromeres of the first, second and third sets in normal manner. 



Figs. 27-29. Three embryos of the same laying showing the failure of the micromeres to overgrow 

 the macromeres, probably the result of dilution of sea water with fresh water. 



Fig. 27. Shows a gastrula with ectomeres and mesomeres (M) forming a cap on the entomeres, 

 though in normal eggs the former would have overgrown the latter at this stage. /n/*„\ a „A 



Fig. 28. View of right side of embryo showing shell gland (ShG), posterior cell plate (ftp) ana 

 velum (7) but with endoderm (End) protruding through blastopore. 



Fig. 29. Embryo similar to the preceding but viewed from anterior pole. 



