Apbil 6, 1906.] 



SCIENCE. 



523 



liminary results were reported from a study 

 of artificial parthenogenesis in the echiuroid 

 Thalassema mellita Conn. At that time it 

 was stated that unfertilized eggs of this 

 worm may be induced to develop into ac- 

 tively swimming trochophores by immersion 

 for a few minutes in dilute solutions of 

 acids, both inorganic and organic. 



Continued and more detailed examina- 

 tion of the material has yielded many addi- 

 tional facts of interest. 



The parthenogenetie development in 

 many cases involves a perfectly normal 

 maturation, a more or less regular cleavage, 

 and the usual processes of differentiation 

 leading up to the formation of the normal 

 larva. 



The unfertilized egg of Thalassema when 

 left in sea-water exhibits no developmental 

 changes, and the germinal vesicle remains 

 intact until the egg dies. After a short 

 exposure to the acid-solutions, however, 

 the egg rounds out upon a return to 

 pure sea-water, and throws off a typ- 

 ical fertilization-membrane. As a rule, 

 both polar bodies are extruded, and 

 sections show that in these eggs the matura- 

 tion-mitoses occur in a normal manner. 

 After maturation, the egg-centrosome and 

 aster disappear; the pronucleus forms 

 from the reduced number of chromosomes 

 and moves to the center of the egg ; the two 

 cleavage-asters with their centers appear 

 de novo and simultaneously at opposite 

 poles of the egg-nucleus ; the first cleavage- 

 figure is then formed, and division of the 

 egg into two equal blastomeres takes place 

 normally. 



In many cases, subsequent cleavages oc- 

 cur in a normal manner, as far as they can 

 be followed, although the rhythm of divi- 

 sion is more or less disturbed; in such 

 cleavages, cytoplasmic division regularly 

 accompanies division of the nucleus, and 

 the mitotic phenomena involved are in all 



respects normal in appearance. The re- 

 duced number of chromosomes (12), how- 

 ever, persists, and has been repeatedly 

 counted even in late blastula- and gastrula- 

 stages. 



Gastrulation consists of the insinking of 

 an entoblastic plate of cells which multiply 

 by division and give rise to the enteron; 

 the latter becomes secondarily divided into 

 stomach and intestine; the oesophagus is 

 formed after gastrulation by an ectodermal 

 invagination which is subsequently placed 

 in communication with the stomach. These 

 processes of differentiation, together with 

 the formation of the prototrochal band and 

 apical flagella, are in all essential respects 

 identical with the corresponding normal 

 events. 



In addition to the cases mentioned in 

 which the normal differentiations are close- 

 ly paralleled, many abnormal processes 

 have also been observed. In some experi- 

 ments only one polar body was extruded, 

 and in others neither was formed; upon 

 sectioning such eggs, it was found that 

 either one or both maturation-mitoses take 

 place well below the surface and without 

 accompanying cytoplasmic division. Cer- 

 tain interesting phenomena are associated 

 with these unusual processes, to which only 

 a reference can be made in this place. 



The formation of large monasters was 

 not infrequently observed, the rays appear- 

 ing and disappearing rhythmically and the 

 chromosomes dividing repeatedly without 

 cleavage of the cytoplasm. 



Nuclear division occurring in the absence 

 of cytoplasmic division often results in a 

 multiplication of chromosomes, which may 

 then be gathered into a single giant nucleus 

 or grouped on a single giant spindle. 



An endless variety of abnormal cleav- 

 ages, similar to those described by others, 

 have been observed; such cleavages fre- 

 quently lead to the formation of ciliated 

 structures, which, however, depart more or 



