62 PROFESSOR E R.A.Y LANRESTER. 



mitively, but arise equally and simultaneously over the whole 

 periphery of the Discogastrula. 



The Discoblastic type of development is found in Birds, 

 Reptiles, Teleostean and Selachian fish, in Cephalopods, some 

 Isopods and Copepods, Scorpions, some Spiders, and a num- 

 ber of Flies. Probably it will be found in Monotremata and 

 Didelphia. 



The Periblastic type (Plate IX) . — This is, perhaps, the 

 most important of any one of the four types distinguished by 

 Haeckel, for here the palingenetic phenomena are most com- 

 pletely disguised. Those embryonic histories which were 

 formerly said to present no yelk-cleavage, or a superficial 

 yelk- cleavage, belong here. The periblastic mode of develop- 

 ment is most common in the Arthropods, in Tracheata as well 

 as Crustacea ; it also appears from KowaleAVsky's observations 

 to occur in some Coelenterata (Alcyonians). Like the amphi- 

 blastic and discoblastic types, the periblastic type exhibits 

 variations, and these throw light on some of the secondary 

 developmental phenomena in the other types. The essential 

 point about the periblastic type is this, that the food-material 

 collects at an early stage of development centrally, so as to be 

 completely enveloped by the formative protoplasm. In many 

 Arthropods this segregation of the two constituents occurs at 

 the time of fertilisation, and we then have what has been 

 called a blastema surrounding granular yelk. This is really 

 the Perimouerula. Later this Perimonerula becomes a Peri- 

 morula by the conversion of its superficial ' blastema ' into a 

 number of cells. This is considered by some authorities 

 (Weissmann, &c.) to be due in the case of the Hexapoda to 

 the free formation of nuclei. 



Good observations on a number of examples are as yet 

 wanting, but we have the excellent researches of Edouard 

 Van Beneden and Emile Bessels, which show that in many 

 Crustacea the many-celled condition of the Perimorula is due 

 to a regular cleavage process. i 



Haeckel is inclined from this to assert as a general rule that 

 the Perimonerula acquires a nucleus, becomes a Pericytula, 

 and then cleaves into two, four, &c., cells, to form the Peri- 

 morula. This process is exhibited in Plate IX, in the case 

 of a Decapod Crustacean (Peneus), studied by Haeckel at 

 Ajaccio. It does not seem, however, at all certain that cases 

 such as that of the Gammarus fluviatilis, described by Ed. 



' I cannot let this notice pass through the press without alluding to the 

 importance of Ed. Van Beneden's excellent work on the composition and 

 signification of the egg, for the whole of the questions which turn on the 

 presence or absence of his (deutoplasm). 



