PLATE XLIV. 

 Abnormalities Found in Nature. 



Figs. 18-26. Show effects probably of pressure; figs. 27-29, of dilute sea water. 



Fig. 18. Four-cell stage; the nucleus is entirely lacking in two of the cells, though sphere and "Zwis- 

 chenkorper" are present. 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. 



Fig. 19. Four-cell stage, showing in the lobes the effects of pressure during the 2d cleavage. 



Fig. 20. Third cleavage showing a lobe in the spindle axis of one cell, the result of pressure. 



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

 spindle axis (<Sp4) is normal, in the former abnormal; spindle one (Spl) lies at a higher level than spindles 

 two and three (Sp2, Sp3). 



Fig. 22. Five macromeres, the two upper ones normal, the three lower ones abnormal; due to a 

 tetraster in the lower cells. 



Fig. 23. Seven macromeres, six of them reaching to the animal pole where they have formed six 

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

 micromere. The nuclei are double or irregular in shape in macromeres IB and ID' and also in the micro- 

 meres derived from them. 



Fig. 24. Seven macromeres, six of them reaching to the animal pole where they have formed six 

 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 IS, a double nucleus in 

 ID 2 and double nuclei and spheres are found in the micromeres 1& 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 first with a 

 central cell containing two nuclei unaccounted for. The micromeres of the first set have divided unequally 

 as in normal eggs. 



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

 direction; the other (3D, 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. 



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

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



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



