EMBRYOLOGY OF A LEPIDOPTEROUS INSECT. 19 



fluid is its vitality, its ability to break up and sub-divide, to develop 

 cellular structure, and to build up tissue from the cells produced by 

 cell-division. After fertilisation, the protoplasmic fluid inside the 

 ovum remains in a homogeneous condition for a certain time ; this 

 varies for different species, but is comparatively constant in the same 

 species. The first change that the protoplasm undergoes is that of 

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

 continues generally with more or less rapidity. The segmentation 

 starts at a point on the surface of the yelk called the " first segmenta- 

 tion 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 separates from the outside cell- 

 wall, and appears to become enveloped in a sac. The blastoderm layer 

 (or layer of segmentation cells) has an elongated ventral plate formed 

 in it, and in this 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 depression, 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. 



Of this, Osborne says : " After the yelk has become surrounded 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 time separating from one another. 

 The inner of these, continuous with the embryo itself, and lying im- 

 mediately over it, is the amnion ; the outer, continuous with the 

 blastoderm surrounding the yelk, is the serous membrane. Two sacs 

 are thus formed, the one within the other, and between them lies the 

 yelk. In the lepidopterous 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 abdominal segments may be noticed, followed 

 by the appearance of the three thoracic segments. 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 

 (each made up of six ocelli), placed one on either side of the second 

 segment of the head, reckoning from the front, are then developed, 

 followed in turn by the ventral prolegs. The inner part of the hypo- 

 blast is absorbed to form the alimentary canal. The cells, now con- 

 tained between the outside wall of the egg and the newly-formed 

 alimentary canal, divide up into clusters, which are gradually differ- 

 entiated 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 



