258 THE EMBRYOLOGY OF THE BLOW-FLY IN THE EGG. 



at the same time sulci appear in the epiblast, separating the 

 larval segments. The fore-head and procephalic lobes rapidly 

 disappear, and the rudimentary cephalic appendages undergo 

 the following modifications. The anterior pair disappear. 

 According to Weismann, they approach each other and form a 

 median outgrowth in the anterior wall of the stomodseum. On 



Fk;. 43. — -The anterior extremity of the Blow-fly embryo in five successive stages, 

 showing the transformation of the mouth organs : /, a side view of the head of 

 the embryo before the segmentation of the coelomic plates, after Weismann ; 

 2, a later stage of the same ; j and 4, still later stages ; j, a ventral view of the 

 head in the same stage as^ ; 6, a ventral view of the head in the mature embryo 

 ready to escape from the egg. 



the same authority the median tooth of the larval mouth 

 armature represents their remains, and is therefore composed 

 of the united mandibles. I am by no means sure that such is 

 its origin, and, as already stated, regard it as the labrum. The 

 second pair of appendages, the maxillae, become parallel with 

 each other, and form the maxillae of the larva. The third pair 



Descrh'tion of Plate XV. 



Two sections through the head of an embryo during the involution of the head discs 



(procephalic lobes). 



Fig I. — A section through the fore-head and maxillae : fh, the fore-head, showing the 

 rudimentary brain and brain vesicles, v ; the single layer of cells enclosing the 

 vesicles, probably becomes the retinal disc ; the cleft, mm, is the orifice of the 

 stomodasum ; iiix, the maxilla ; t^, the maxillary disc ; s g, salivary duct. The 

 cellular mass, <•, is probably part of the rudimentary pharynx. 



Fii;. 2. — A section tlirough the paracephala (procephalic lobes) of the same embryo : 

 (/ V, dorsal vessel ; »ius, rudimentary muscles ; tg, ganglion of the ventral chain ; 

 stf stomodceum ; i^, antennal, and t-, optic imaginal discs ; s g, salivary duct. 



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