NOTES ON THE DEVELOPMENT OF THE ARANEINA. 173 
procephalic lobe. I cannot accept the validity of this argu- 
ment ; though I am glad to find myself in^ at any rate, 
partial harmony with the distinguished French embryologist 
as to the facts. Balbiani denies for this stage the existence 
of a caudal lobe. There is certainly, as is very well shown 
in my longitudinal sections, a thickening of the blastoderm in 
the caudal region, though it is not so prominent in surface 
views as the procephalic lobe. 
A transverse section through an embryo at this stage (PI. 
XX, fig. 1^) shows that there is a ventral plate of somewhat 
columnar cells more than one row deep, and a dorsal portion 
of the blastoderm formed of a single row of flattened cells. 
Every section at this stage shows that the inner layer of 
cells of the ventral plate is receiving accessions of cells from 
the yolk, which has not to any appreciable extent altered 
its constitution. A large cell, passing from the yolk to the 
blastoderm, is shown in fig. 12 at y. c. 
The cells of the 'central plate are now divided into two 
distinct layers. The outer of these is the epihlast, the 
inner the mesohlast. The cells of both layers are quite 
continuous across the median line, and exhibit no trace of a 
bilateral arrangement. 
This stage is an interesting one on account of the striking 
similarity which (apart from the amnion) exists between a 
section through the blastoderm of a spider and that of an 
insect immediately after the formation of the mesoblast. 
The reader should compare Kowalevsky’s (^ Mem. Acad. 
Petersbourg,’ vol. xvi, 1871) fig. 26, PI. IX with my fig. 
12. The existence of a continuous ventral plate of meso- 
blast has been noticed by Barrois (p. 532), who states that 
the two mesoblastic bands originate from the longitudinal 
division of a primitive single band. 
In a slightly later stage (PI. XIX, fig. 3 a and 3 h) six 
distinct segments are interpellated between the procephalic 
and the caudal lobes. The two foremost, ch and pd (especially 
the first), of these are far less distinct than the remainder, 
and the first segment is very indistinctly separated from 
the procephalic lobe. From the indistinctness of the 
first two somites, I conclude that they are later formations . 
than the four succeeding ones. The caudal and procephalic 
lobes are very similar in appearance, but the procephalic lobe 
is slightly the wider of the two. There is a slight protuber- 
ance on the caudal lobe, which is possibly the remnant of 
the cumulus. The superficial appearance of segmentation 
is produced by a series of transverse valleys, separating 
raised intermediate portions which form the segments. The 
