THE LIFK-HISTORY OF A LEPIDOPTEROUS INSECT. 213 



tmie ; this varies for different species, biit is comparatively constant in tlie 

 same species. The lirst change that the protoplasm undergoes is that of 

 the ordinary yelk-segmentation, but, once this is set up, development con- 

 tinues generally with more or less rapidit3\ The segmentation starts at 

 a point on the surface of the yelk called the " first segmentation nucleus," 

 and this nucleus undergoes cell-division in such a manner as to form 

 a superficial blastodermic layer ; side by side with this process of 

 segmentation, the yelk sejijarates from the outside cell- wall and appears 

 to become enveloped in a sac. The blastodermic layer (or layer of 

 segmentation cells) has an elongated ventral plate formed in it, and in 

 tliis the development of the embryo commences. This ventral plate 

 broadens anteriorly, but the posterior part is divided transversely into 

 segments. This development is at once followed up by the formation 

 of a longitudinal depression, the outer sac gradually enclosing this 

 depression on either side until at last the opposite sides of the epiblast, 

 or outside layer of cells undergoing segmentation, unite over the de- 

 pression, leaving it as a longitudinal tube. This becomes detached as a 

 solid cellular mass, which splits into two longitudinal (mesoblastic) bands. 

 At this period it would appear that the amnion is formed. Dr. Osborne 

 writes: — (Science Gossij), \ol. xxi.) "After the yelk has become sur- 

 rounded by the growth of cells called the blastoderm and after the germinal 

 stripe, or foundation of the embryo has been differentiated along one side 

 of this blastoderm, a double fold of the latter grows up all round the cir- 

 cumference of the germinal stripe and finally closes in over it, the edges 

 of the fold fixing together and the two layers (of blastoderm) of which it 

 is composed, at the same tinle separating from one another. The inner 

 of these continuous with the embryo itself, and lying immediately over 

 it, is the amnion ; the outer, continuous with the blastoderm surround- 

 ing the yelk, is the serous membrane. Two sacs are thus formed, the 

 one within the other, and between them lies the yelk. In the lejji- 

 dopterous egg the yelk next finds its way into the space between the 

 amnion and the serous membrane, flowing over the former and depressing 

 it and the embryo beneath it till both are completely submerged in yelk, 

 and consequently hidden from view." After this the mesoblastic bands 

 become divided into somites, and the first traces of the ventral segments 

 may be noticed, followed by the appearance of the three thoracic seg- 

 ments. The somites coalesce and the common body- cavity thus enclosed 

 is called the coelom. The three thoracic segments bear legs. The head, 

 which appears to be formed of four segments, and the eye-spots (of 

 which there are two clusters of six, placed at the base of the 3rd seg- 

 ment, reckoning from behind forwards) are then developed, followed in 

 turn by the ventral prolegs. The inner part of the hypoblast is ab- 

 sorbed to form the alimentary canal. The cells now contained between 

 the outside egg-wall and the newly-formed alimentary canal divide 

 up into clusters, which are gradually dift'erentiated into the various 

 internal organs. The first of these to be formed is the dorsal vessel, 

 which is so called because it is placed in the dorsal part of the larva ; 

 this corresponds with the heart of the higher animals. The otlier 

 organs gradually undergo differentiation, and the mouth organs also 

 become developed. At this period of development faint pulsations of 

 the dorsal vessel are discernible. The separation of the alimentary 

 canal into an oesophagus, a widened sac or stomach, and another con- 

 tracted tube or intestine is clearly discernible, whilst the outer proteid 



