NOTES ON THE DEVELOPMENT OF THE ARA.NEINA. 89 



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. 

 IX, fig. 12) 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 ventral plate are now divided into two 

 distinct layers. The outer of these is the epiblast, 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 mesohlast. 

 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- 

 hlast 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 (PL VIII, 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 



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