CHAPTER XVI. 



NEMATELMINTHES AND ACANTHOCEPHALA. 



NEMATELMINTHES 1 . 



Nematoidea. Although the ova of various Nematodes have formed 

 some of the earliest, as well as the most frequent objects of embryologi- 

 cal observation, their development is still but very imperfectly known. 

 Both viviparous and oviparous forms are common, and in the case of 

 the oviparous forms the eggs are usually enveloped in a hard shell. 

 The segmentation is total and nearly regular, though the two first 

 segments are often unequal. The relation of the segmentation spheres 

 to the germinal layers is however only satisfactorily established 

 (through the researches of Biitschli (No. 383)) in the case of Cucullanus 

 elegans, a form parasitic in the Perch 2 . 



The early development of this embryo takes place within the body 

 of the parent, and the egg is enveloped in a delicate membrane. After 

 the completion of the early stages of segmentation the embryo acquires 

 the form of a thin flat plate composed of two layers of cells (fig. 166 

 A and B). The two layers of this plate give rise respectively to the 

 epiblast and hypoblast, and at a certain stage the hypoblastic layer 

 ceases to grow, while the growth of the epiblastic layer continues. 

 As a consequence of this the sides of the plate begin to fold over 

 towards the side of the hypoblast (fig. 166 D). This folding results in 

 the formation of a remarkably constituted gastrula, which has the 

 form of a hollow two-layered cylinder with an incompletely closed 

 slit on one side (fig. 166 E, bl.p). This slit has the value of a 

 blastopore. It becomes closed by the coalescence of the two edges, a 



1 The following classification of the Nematoda is employed in this chapter : 



f Asearidse. 

 Strongylidfe. 



T XT n +^Ar> f . { Trichinidse. H. Gordioidea. 



I- Nematoidea. ^ Fi i ari da3. ill. Chsetosomoidea. 



I Mermithidffi. 

 {_ Anguillulidte. 



2 The ova of Anguillula aceti are stated by Hallez to undergo a similar development 

 to those of Cucullanus. 



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