92 REVIEW. 



spot, just between the mandibles, is so deep that in transverse 

 section it has all the appearance of the medullary groove of a 

 cliick at the end of the first day of incubation. Immediately be- 

 hind the abdomen, between it and the carapace (B), is an insig- 

 nificant swelling (N.) marking the position of the heart. 



A longitudinal section (fig. 1 , woodcut) shows that the archen- 

 teron or midgut {mg.) has now become a very considerable cavity, 

 extending, forwards and backwards, beyond the limits of the em- 

 bryonic area ; its cavity is filled with a coagulated substance of a 

 finely granular appearance. The foregut {fg.) is still a blind pouch 

 separated from the archenteron by a layer of mesoderm, but the 

 hindgut [kg.) is completely formed, and has established a com- 

 munication with the midgut at some distance from its posterior 

 end. The cells lining the latter cavity are, on its neural aspect, 

 of the ordinary columnar character, but on its hfemal aspect — 

 that is, on the side turned towards the main mass of yolk — they 

 have increased greatly in size, and are very full of ingested 

 vitelline spheres ; they have, in fact, by the method already 

 described, eaten their way for a considerable distance into the 

 food-yolk, and attained a correspondingly gorged and hyper- 

 trophied appearance. 



The optic fossae (PI. VI, fig. 6, V) have become, in this stage, 

 deep invaginations, lined with large columnar cells, and in connec- 

 tion with two longitudinal cords, formed by the inward prohfera- 

 tion of the cells forming the lateral parts of the medullary groove. 

 These cords — the commencement of the central nervous system — 

 can be traced as far back as the caudal fold ; anteriorly they curve 

 round the gullet, and unite in front of it. Their pra^-oral por- 

 tions, formed by lateral ingrowths of the ectoderm — a virtual 

 not an actual medullary groove — constitute the lateral cords 

 (Seitenstrange) from which part of the supra-oesophageal 

 ganglion is produced. The remainder of the ganglion is formed, 

 partly from the ingrowth of cells from the optic fossre, and 

 partly from a row of cells (Mittelstrang) lining a groove which 

 is formed between the lateral cords at the period when the ambu- 

 latory limbs make their appearance. 



The origin of the secondary mesoderm cells, mentioned above, 

 is well seen in this stage, in Reichenbach's figure of a transverse 

 section through the optic fossae. A smaller portion of this section 

 is reproduced in fig. 6 (woodcut), which represents the upper 

 (neural) ends of the endodermal cells from the neural side of 

 the midgut. The protoplasm is mostly accumulated round the 

 nucleus (w.) at the upper end of the cell, the remainder of which 

 is filled with vacuolated deutoplasm. A very large proportion 

 of the cells have two or three nuclei, and in some {e.g. the 

 middle cell in fig. 0) the division process has been so rapid that 



