18-26 



PLATE XLIV. 



Abnormalities Found in Nature. 

 robably of pressure: figs. 27-29, of dilu 



Fig. 18. 



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 



arance 



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 



normal, in the former abnormal 



two and three (Sp2, Sp3). 



Fig. 22. Five macror 

 tetraster in the lower cells. 



Fig. 23. Seven macri 

 micromeres of the first set. 

 micromere. The nuclei ar( 

 meres derived from them. 



normal, the three lower ones abnormal 



formed 



form 



gular in shape in macromeres IB and ID' and also in the micro- 



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

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

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



and double nuclei and spheres 

 Fig. 25. Six macromeres (2; 



first with 



micromeres 



unaccounted 



normal 

 Fig. 26. 



k UlltIUUlCl\,0 V/X V"X> ^v,~w~~ 



for. The micromeres of the first set have divided unequally 



™. «,. Three macromeres, one of them (D) giving rise to a micromere , d : the first set in ^normal 

 direction; the other (3S, 3C) have produced micromeres of the first, second and third f^^ ~ 

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

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



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

 irfi in normal eggs the former would have overgrown the latter at this stage. 



showing shell gland (ShG), posterior cell plate K^V) 



'i 



Fig. 28. 



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

 Fig. 29. Embryo similar to the preceding but viewed from an 



