Dipneumones.
III. Araneina.
[180] The origin of the hypoblast cells, if such these cells are, is obscure. Metschnikoff doubtfully derives them from the blastoderm cells; from my investigations on Spiders it appears to me more probable that they originate in the yolk.
[181] The exact fate of the three original segments is left somewhat obscure by Metschnikoff. He believes however that the anterior segment forms the procephalic lobes, the posterior probably the telson and five adjoining caudal segments, and the middle one the remainder of the body. This view does not appear to me quite satisfactory, since on the analogy of Spiders and other Arthropoda the fresh somites ought to be added by a continuous segmentation of the posterior lobe.
[182] Various views have been put forward by Claparède and Balbiani about the position and significance of the primitive cumulus. For a discussion of which vide self, No. 440.
[183] For further details vide self, No. 440.
[184] The primitive streak of Vertebrates, as will appear in the sequel, has no connection with the medullary groove, and is the rudiment of the blastopore.