The oral invagination appears nearly coincidently with the first formation of segments at the front end of the groove between the lateral nerve cords, and the anal invagination appears slightly later at the hindermost end of the ventral plate.
The Malpighian bodies arise as two pairs of outgrowths of the epiblast of the proctodæum, whether solid at first is not certain. The subsequent increase which usually takes place in their number is due to sproutings (at first solid) of the two original vessels.
The glandular walls of the mesenteron are formed from the hypoblast; but the exact origin of the layer has not been thoroughly worked out in all cases. In Hydrophilus it is stated by Kowalevsky (No. [416]) to appear as two sheets split off from the lateral masses of mesoblast, which gradually grow round the yolk, and a similar mode of formation would seem to hold good for Apis. Tichomiroff (No. [420]) confirms Kowalevsky on this point, and further states that these two masses meet first ventrally and much later on the dorsal side. In Lepidoptera, on the other hand, Hatschek finds that the hypoblast arises as a median mass of polygonal cells in the anterior part of the ventral plate. These cells increase by absorbing material from the yolk, and then gradually extend themselves and grow round the yolk.
Dohrn (No. [408]) believes that the yolk cells, the origin of which has already been spoken of, give rise to the hypoblastic walls of the mesenteron, and this view appears to be shared by Graber (No. [412]), though the latter author holds that some of the yolk cells are derived by budding from the blastoderm[174].
From the analogy of Spiders I am inclined to accept Dohrn’s and Graber’s view. It appears to me probable that Kowalevsky’s observations are to be explained by supposing that the hypoblast plates which he believes to be split off from the mesoblast are really separated from the yolk.
It will be convenient to add here a few details to what has already been stated as to the origin of the yolk cells. As mentioned above, the central yolk breaks up at a period, which is not constant in the different forms, into polygonal or rounded masses, in each of which a nucleus has in many instances been clearly demonstrated although in others such nuclei have not been made out. It is probable however that nuclei are in all cases really present, and that these masses must be therefore regarded as cells. They constitute in fact the yolk cells. The periphery of the yolk breaks up into cells while the centre is still quite homogeneous.
The hypoblastic walls of the mesenteron appear to be formed in the first instance laterally ([fig. 189] B and C, hy). They then meet ventrally ([fig. 185] A and B), and finally close in the mesenteron on the dorsal side.
The mesenteron is at first a closed sack, independent of both stomodæum and proctodæum; and in the case of the Bee it so remains even after the close of embryonic life. The only glandular organs of the mesenteron are the not unfrequent pyloric tubes, which are simple outgrowths of its anterior end. It is possible that in some instances they may be formed in situ around the lateral parts of the yolk.
In many instances the whole of the yolk is enclosed in the walls of the mesenteron, but in other cases, as in Chironomus and Simulia (Weismann, No. [430]; Metschnikoff, No. [423]), part of the yolk may be left between the ventral wall of the mesenteron and the ventral plate. In Chironomus the mass of yolk external to the mesenteron takes the form of a median and two lateral streaks. Some of the yolk cells either prior to the establishment of the mesenteron, or derived from the unenclosed portions of the yolk, pass into the developing organs (Dohrn, 408) and serve as a kind of nutritive cell. They also form blood corpuscles and connective-tissue elements. Such yolk cells may be compared to the peculiar bodies described by Reichenbach in Astacus, which form the secondary mesoblast. Similar cells play a very important part in the development of Spiders.
Generative organs. The observations on the development of the generative organs are somewhat scanty. In Diptera certain cells—known as the pole cells—are stated by both Metschnikoff (No. [423]) and Leuckart to give rise to the generative organs. The cells in question (in Chironomus and Musca vomitoria, Weismann, No. [430]) appear at the hinder end of the ovum before any other cells of the blastoderm. They soon separate from the blastoderm and increase by division. In the embryo, produced by the viviparous larva of Cecidomyia, there is at first a single pole cell, which eventually divides into four, and the resulting cells become enclosed within the blastoderm. They next divide into two masses, which are stated by Metschnikoff (No. [423]) to become surrounded by indifferent embryonic cells[175]. Their protoplasm then fuses, and their nuclei divide, and they give rise to the larval ovaries, for which the enclosing cells form the tunics.