Fig. 171. Head of an embryo Peripatus. (From Moseley.)
The figure shews the jaws (mandibles), and close to them epiblastic involutions, which grow into the supra-œsophageal ganglia. The antennæ, oral cavity, and oral papillæ are also shewn.
I have been able to make a few observations on the internal structure of the embryos from specimens supplied to me by Moseley. These are so far confined to a few stages, one slightly earlier, the others slightly later, than the embryo represented in [fig. 168] B. The epiblast is formed of a layer of columnar cells, two deep on the ventral surface, except along the median line where there is a well-marked groove and the epiblast is much thinner ([fig. 172]).
The ventral cords of the trunk are formed as two independent epiblastic thickenings. In my earlier stage these are barely separated from the epiblast, but in the later ones are quite independent ([fig. 172], v.n), and partly surrounded by mesoblast.
The supra-œsophageal ganglia are formed as thickenings of the epiblast of the ventral side of the procephalic lobes in front of the stomodæum. They are shewn at l in [fig. 170]. The thickenings of the two sides are at first independent. At a somewhat later period an invagination of the epiblast grows into each of these lobes. The openings of these invaginations extend from the oral cavity forwards; and they are shewn in [fig. 171][161]. Their openings become closed, and the walls of the invaginations constitute a large part of the embryonic supra-œsophageal ganglia.
Similar epiblastic invaginations assist in forming the supra-œsophageal ganglia of other Tracheata. They are described in the sequel for Insects, Spiders and Scorpions. The position of the supra-œsophageal ganglia on the ventral side of the procephalic lobes is the same as that in other Tracheata.
Fig. 172. Section through the trunk of an embryo of Peripatus. The embryo from which the section is taken was somewhat younger than fig. 171.
sp.m. splanchnic mesoblast. s.m. somatic mesoblast. mc. median section of body cavity. lc. lateral section of body cavity. v.n. ventral nerve cord. me. mesenteron.