228 LILIAN SHELDON. 



would doubtless be an easy and rapid mode of formation. It 

 is not until after the peripheral nutriment has been mainly 

 absorbed, so that the amount of space within the egg-shell is 

 increased, that the posterior part of the body loses its close 

 adherence to the anterior, and the embryo begins to straighten 

 out. Also, apart from the space occupied by it, the peripheral 

 yolk would probably act as a resistant force against a normal 

 lengthways growth of the embryo. Ganin, in his account of 

 the development of Platygaster, referred to above, describes a 

 process whereby a result somewhat similar to that effected in 

 P. novse-zealandise is brought about: — the embryo after 

 the segmentation is completed consists of a solid mass of 

 cells, the peripheral layer being distinguished from the 

 central mass by their more columnar form. An invagina- 

 tion of the ventral surface then occurs, forming a deep trans- 

 verse fissure extending about half way across the embryo, and 

 dividing it into an anterior cephalothoracic and a posterior 

 caudal portion. So that at this stage the embryo has much 

 the same characters as that of P. novse-zealandise after the 

 anterior and posterior regions of the body have acquired their 

 own ventral walls and have become definitely distinct from one 

 another. The stage in which the two are separated only by a 

 single septum does not appear to possess any parallel in the 

 development of Platygaster. But, apart from this, the forma- 

 tion in situ of an embryo doubled upon itself from a primi- 

 tively single and solid mass is very remarkably similar in the 

 two cases. It would appear to have been acquired as a simple 

 process, the conditions being, in the fact of the enclosure of the 

 embryo in a peripheral layer, somewhat similar in both ova. 



Origin of the Endoderm. — As I said in my remarks on 

 the segmentation, the first endodermal nuclei seem to arise by 

 a process of free nuclear formation. The same may perhaps 

 be the case with the later endodermal nuclei, since in no case 

 have I found any trace in them of karyokinetic figures, which 

 latter are extremely common in all the other nuclei. At the 

 stage before the embryo is definitely formed, when the flattened 

 protoplasmic plate is present along one side of the egg, there 



