378 SUMMARY OF CURRENT RESEARCHES RELATING TO 



The germinal band forms a narrow elongated layer of low cylindrical 

 cells, which broadens out at the future head-end into two large lateral 

 lobes. Segmentation of this band does not take place till a relatively 

 late period, though it is, at a very early stage, remarkable for the 

 centralization of its cells around certain points which are the centres of 

 the formation of the future extremities. 



In an early stage of development all the rudiments of the extremi- 

 ties have a similar structure ; they form simple outpushings of the 

 ectoderm, the cavities of which are invested by rounded mesodermal cells. 

 The antennsB are pretty long ; the mandibles are very small ; the pro- 

 portionately very long thoracic feet begin very early to exhibit signs of 

 segmentation. The first pair of ventral appendages are considerably 

 longer than the others, though they have exactly the same structure. 



The succeeding changes in the form of the embryo, in addition to 

 its general growth and the development of the lateral parts of the body, 

 chiefly consist in a change in the form and internal structure of the 

 first abdominal appendage ; instead of becoming longer they become 

 broader, while the base narrows ; the mesodermal cells appear to wander 

 into the body-cavity of the embryo ; at any rate, the number in the first 

 abdominal appendage gradually decreases. This becomes pyriform in 

 shape, and is only attached to the body by means of a thin and small 

 stalk. The greater part consists of very long and narrow, almost 

 spindle-shaped, ectodermal cells, which form, by their diverging distal 

 ends, the surface of their appendage, while their proximal ends converge 

 towards the stalk. They lie very close to one another, and there is no 

 internal cavity in this part of the altered extremity, while in the axis of 

 the stalk there is only a canal leading into the body-cavity. In the 

 later stages of development the organ disappears in a manner which 

 has not yet been worked out. 



Patten is not correct in saying that all the other abdominal appen- 

 dages disappear rapidly ; while the second to the ninth disappear as 

 changes occur in the hinder part of the body, the tenth and eleventh 

 undergo further development ; the eleventh form the future cerci, and 

 the tenth pair forms small appendages which persist throughout life in 

 the male, but become lost in the female. The cerci do not become 

 jointed till the embryo leaves the egg. 



The development of the dorsal region is next briefly described, and 

 it is stated that the tracheee do not begin to be formed as invaginations 

 of the ectoderm until it is completed. It is in consequence of this late 

 appearance of the tracheae that the embryo of Blatta is so especially 

 adapted for the study of the rudimentary appendages, for in other 

 insects the stumps of these organs may be easily confused with the 

 ridges of the stigmata. 



In the next place the author advances some general considerations. 

 He first insists on the fact that the embryo described by him had 

 eighteen pairs of well-developed appendages, nine of which in the male, 

 and eight in the female persist throughout life. All these appendages 

 have at first an altogether similar structure, and their cavities com- 

 municate with the corresponding cavities of the somites. He is led to 

 believe that the insects are derived from ancestors that were polypodous 

 and homopodous, and probably like Scolojpendrella ; that these ancestors 

 did not live in water, but led, at most, an amphibious life, and that, in 

 any case, they had nothing to do with the Crustacea. There does not 



