f58 Embryonic Development of Chironomus 



Methods. 



Fertiliza- 

 tion and 

 segmenta- 

 tion. 



within tlie vitelline membrane is filled with a brownish 

 yellow 5^olk, containing a multitude of fat-globules 

 of various sizes (fig. 117). We can indistinctly make 

 out a thin superficial layer of clear protoplasm, which 

 becomes more evident in stained sections, where it is 

 seen to send out many thread-like extensions into the 

 yolk. 



The ovarian q,^^ of the pupa or imago is enclosed 

 in a follicle of the egg-tube (fig. 83), which is lined, until 

 the eggs are almost ready to be laid, by a scanty 

 epithelium. The follicle in an early stage encloses 

 a true egg-cell, whose nucleus is the germinal vesicle, 

 and also several nutritive cells, which dwindle as the 

 yolk increases (see p. 113)- 



The posterior end of the ovum is the first to pass into 

 the oviduct, and it is probable that Hallez' law of 

 orientation, viz. that the ends and faces of the ovum are 

 placed similarly to the corresponding parts of the parent, 

 obtains here as in all insects which have been investi- 

 gated. 



In order to study the early embryonic stages to the 

 best advantage, special preparation is necessary. The 

 following method we have found to succeed : — The egg- 

 chain is killed with hot 30°/^ alcohol, half saturated with 

 corrosive sublimate. It is then gradually transferred 

 to absolute alcohol, and subsequently to chloroform and 

 melted paraffin. Sections are cut by the microtome, and 

 stained on the slide by Heidenhain's haematoxjdin 

 method. The observation of living embryos should not 

 be omitted, and much may be learnt by those who are 

 unable to prepare sections at all. 



.Before fertilization the egg-nucleus travels to the 

 surface and divides. The two polar bodies thus formed 

 are not ejected, but break up within the egg. After 

 fertilization male and female pronuclei form in the 



