82 BULLETIN OF THE 
cells are derivatives from the yolk-cells, and first appear just before the 
reversion of the embryo begins. ‘They are abundant along the sides of 
the body, and about the cesaphagus as well as in the dorsal region. 
The yolk during this period is somewhat changed from its early char- 
acteristics. The corpuscles are undergoing disintegration, and are much 
vacuolated, which gives them in certain regions a spongy appearance. 
The nuclei of the yolk-cells, while they have increased in number, are 
smaller and angular (often triangular) in outline. 
5. The period from reversion to hatching. — Few surface changes of 
importance are necessary to convert the embryo of the period just de- 
scribed into the adult. The following are the most obvious: The embryo 
becomes more closely flexed upon itself (Pl. II. fig. 11), and the constric- 
tion which separates the abdomen and the cephalo-thorax is formed. At 
least two pairs of provisional appendages are modified into as many 
large spinning mammille.* In addition to these two large pairs there 
is a pair of smaller median mammille, the origin of which I have not 
traced. The remnant of the tail persists for some time as a post-anal 
knob; upon the ventral surface appear the infoldings, from which are 
formed the trachez, and also those of the generative organs; upon the 
head the eyes make their appearance. Two or three days before hatch- 
ing the embryo begins to unroll, and undergoes a moult ; at the time of 
hatching it is completely straightened. 
I shall now proceed, after this general account of the more important 
embryonic stages, to the consideration of the development of separate 
organs and sets of organs. 
Ill. — Organogeny. 
In the present paper only the following organs will receive attention : 
(1) the alimentary tract, including stomodeum, pharynx, stomach, mid- 
intestine, stercoral pocket and rectum; (2) the eyes; and (3) the lungs. 
* Balfour (’80, p. 183) has stated: ‘The four rudimentary appendages have 
disappeared, unless, which seems to me in the highest degree improbable, they 
remain as the spinning mammille.” Notwithstanding his doubt, I think I have 
traced the development of two pairs directly into the mammille. The mammille, 
therefore, are appendages of abdominal somites, homodynamic with the cephalo- 
thoracic appendages, and there are consequently six somites condensed into the 
space between the posterior pair of mammille and the anus. Upon the ventral face 
the evidences of this are early obliterated, but upon the dorsal surface the poste- 
rior somites are recognizable by the arrangement of the longitudinal muscles, at 
least as late as the stage represented in Fig. 70, Pl. XI. 
