572 SUMMARY OF CURRENT RE8EAJ10HES RELATING TO 



daughter-elements on the dislocation of the equatorial plate is the result 

 of a circulating streaming ; lie supposes that the spindle-shaped fibres 

 form a continuous coil, and that a stream of fluid circulates in them in a 

 definite direction. If wc suppose that the daughter-elements pass along 

 the fibres and are moved by the stream, it follows that they must separate 

 from one another in opposite directions. The changes in the form and 

 position of the spindles are believed to be the result of the mechanical 

 action of the fluid moving away from the poles. If the asters arise 

 primarily their origin is independent of the direction in which the 

 stream of nutrient fluid traverses the cell, and the spindles are developed 

 at right angles to it. The changes in the position of the nucleus are 

 due to the same cause. 



The formation of the coil and the arrangement of the equatorial plate 

 are believed to be the result of protoplasmic streams which traverse the 

 nucleus in a definite direction. The achromatic substance is considered 

 to be the active element in karyokinesis, the phenomena of which cannot 

 be exi^lained by supposing the existence of opposing forces. Division 

 of the protoplasm is looked upon as a purely mechanical process ; the 

 constriction and separation of dividing animal-cells being a simple 

 mechanical consequence of the elongation of the nuclear spindle. 



Decrease of Weight in Winter Pupae of Pontia brassicae.* — Herr 

 F. Urech has made a number of elaborate observations on the weight of 

 the pupae of Pontia hrassicse. He finds that this weight steadily 

 diminishes. If the temperature surrounding the pupae be kept constant, 

 the decrease is increased towards the end of the pupal stage, and 

 especially so a few days before escape ; if the temperature be raised 

 moderately, the duration of the pupal stage diminishes ; dry air has an 

 abbreviating influence on the duration of this stage. 



Development in Egg of Musca vomitoria.t — Dr. A. Voeltzkow 

 has a preliminary communication on the development of Musca vomiioria. 

 The blastoderm is formed simultaneously over the whole periphery of 

 the egg, and no cells remain internally. The polar cells lie at the 

 hinder pole of the egg, and by their presence push the cells of the 

 blastoderm inwards, so that a conical process projects into the interior 

 of the egg. From this cone blastoderm-cells break oft", which wander 

 into the interior, and form the so-called yolk-cells ; these, in Musca, 

 mainly serve to break up the yolk. 



The formation of the germinal layers commences with an invagina- 

 tion of the blastoderm on the whole of the ventral surface, and an 

 almost completely closed tube is so formed. The germ-stripes are drawn 

 over on the dorsal surface by the development of dorsal folds. The 

 three layers arise by the constriction and subsequent flattening out of 

 the tubes. The rudiment of the hind-gut now appears as an invagina- 

 tion of the dorsal ectoderm in the hinder third of the egg. The 

 oesophageal invagination does not appear till somewhat later. The 

 amnion is formed simultaneously with the rudiment of the hind-gut, 

 and later on it forms the greater part of the back of the embryo. The 

 polar cells wander on to the dorsal surface, and pass into the hind-gut ; 

 their later fate has not yet been made out. 



The mid-gut is formed by two lateral thickenings of the endoderm, 

 just behind the blind end of the oesophagus ; the lateral pads so 



• Zool. Anzeig., xi. (1S88) pp. 205-12. t Ibid., pp. 235-6. 



